Identifying Titles of the Writings:
Here is a brief and simplified explanation of the identifying titles of the Writings. Some of these expressions have been given to certain items by the Authors as identifying titles. In some instances one item has been referred to by two or more such titles.
Kitáb (pl. Kutub) is an Arabic word that has many meanings and connotations. It refers to what is written, a letter, a book, a risalih, a safihih and also it means command, obligation (what has to be followed or done). However usually, specially nowadays, it is used for book.
Sahífih (pl. Suhuf or Saha'if) is a letter (a risalih), a book, a booklet, a sheet of paper
Risalih (pl. Rasa'il) is a message written to certain addressee/s. It may be a letter or a long letter in the form of a small book ( a booklet).
Lawh (pl. Alwah) is a surface which is wide open to be written on or having a statement already written on. It may be of such substances as stone, bone, wood, metal or paper.
Towqi' (pl. Tavaqi' or toiwqi'at) is putting a special sign on something such as the signature on a letter or a decree. It may also mean writing some additional notes at the end of a letter or a book. It is used as an expression meaning decree, or a letter written in behalf of the king or a high ranking person and signed by him.
Dast Khat is used for a message or letter written by a high ranking person usually in his or her own handwriting.
Súrih means high rank, honor, dignity, merit, excellence. It is used as an expression identifying a chapter or an independent part of a sacred book like the independent sections of Qur'an.
Khutbih is a sermon. Usually it is the sermon in praise of God testifying to His oneness and His attributes which is written at the opening of a book, a risalih or even a letter. Sometimes Khutbih may be long and it may be an independent item by itself.
In the Bahá'í Writings in Persian and Arabic all of these identifiers have been used More specifically and usually they are used for the following purposes:
All the Writings of Bahá'u'lláh, unless otherwise named by Himself, are referred to as Lawh (pl. Alwah). This expression has been used by Bahá'u'lláh Himself too. Some of the Writings of the Báb is also called Lawh such as Lawh-i-Hayakil and Lawh-i-Hurúfát.
All the Writings of Báb to specific addressee/s, unless otherwise named by Himself, are called Tawqi', such as His letters to Muhammad Sháh.
The letters written by `Abdu'l-Bahá, unless otherwise referred to by Himself, are named Maktoob (pl. Makatib) by Himself.(Maktúb is an Arabic word meaning what is written and is used to indicate a letter). The title of Lawh is often used (idiomatically and as a sign of rank and respect) for the letters written by `Abdu'l-Bahá.
The letters written by Shoghi Effendi are referred to as Towqi' (pl. Tavaqi' or Towqi'at). This is because He usually was dictating his letters, then adding a few words or lines at the end in his own handwriting and sign them. His general/universal messages are called Payam (meaning message).
Dast Khat is used for the letters written by the Greatest Holy Leaf.
Messages of the Universal House of Justice are called Payam and its letters Namih (letter).
I hope this brief explanation would help non-Persian speaking friends to differentiate between these various expressions and titles.
On "Pure" Persian versus "Pure" Arabic:
"Pure Persian" is a rather new term. Starting in the middle of the nineteenth century some Persian modernists and nationalists popularized the slogan of "pure Persian" as a call for return to pre-Islamic Persia and Persian. This is a movement that has continued up to this time. As Dr. Stockman observed it seeks to rid Persian from words of Arabic root. This has remained as a hot issue for over a century. As Shoghi Effendi wrote in some of his messages to the friends in Iran and as repeatedly emphasized by impartial and learned Iranian linguists this is a futile effort because in Persian language "pure Persian" and words of Arabic origin have blended together in such a way that they cannot be separated from each other. Many of the Arabic words have become Persianized. They are used in Persian with new meanings quite different from their meanings in Arabic. Persians language has one of the richest and most valuable literature based on this unique blend.
There is no such situation in the case of Arabic. However amongst the Arabs are also elements who argue that Arabic should be kept pure and there has been attempts to coin words in Arabic for modern terms. This is also true in the case of many other languages such as French and German.
Due to the nature of the Persian language and the style and language of the revelation of Bahá'u'lláh in Arabic it is next to impossible to render His Arabic texts into Persian without sacrificing both some substance and nearly most of its literary characteristics. While it is permitted to render the contents of the Arabic texts into Persian for those who does not know Persian and would like to know the content exact authoritative translation into Persian was not approved by the Guardian. It should be remembered that mastering Persian language requires some basic knowledge of Arabic. As Dr. Stockman explained, it is like knowing some Latin to be able to master English language.
Some of the adherents of "Pure Persian" wrote to Abdu'l-Bahá raising the same question of why certain Bahá'í texts are revealed in Arabic and why pure Persian was not used throughout the revelation. There are Tablets of Abdu'l-Bahá in response to such questions. In addition to the Tablets and prayers that Bahá'u'lláh and Abdu'l-Bahá have written in so called pure Persian, Abulfadl Gulpayegani, most famous Bahá'í scholar and teacher, has also written many items in pure Persian. As a matter of fact the best specimen of the texts in pure Persian are those revealed by Bahá'u'lláh, written by Abdu'l-Bahá and composed by Gulpayegani!
The fact is that the Arabic texts in Bahá'í scripture are written in a style that Persians with elementary knowledge of Arabic can read and understand them. After all we believe in and follow the guidance given by Bahá'u'lláh and His authorized infallible Interpreters. Thus even the above-mentioned explanations are not needed to convince us to follow their advice!
By the way no one has yet measured the volume of the revelation of Bahá'u'lláh in Persian and in Arabic to compare them and find out which one is more voluminous. The fact is that most of the Tablets addressed to individuals are either in Persian or in a mixture of Persian and Arabic. That is some sections are in persian and some parts or sentences are in Arabic. However the texts on laws and ordinances are mostly in Arabic. The tablet of Ahmad in persian is not the same as the Tablet of Ahmad in Arabic. These are two different tablets revealed in honor of tow different individuals. You can read about them in Taherzadeh's Revelation of Bahá'u'lláh, Vol. II.