I want to know the difference in meaning when I say 'the envelope provided' and 'the provided envelope'. please tell me the difference. If there is no difference in meaning, why do they use these two different expressions?
There is no difference in meaning between the two sentences.
In the first sentence, the participle provided is postnominal: it is after the noun it modifies. In the second sentence, it is prenominal: before the noun it modifies, where adjectives usually are placed in English.
Participles are more likely to appear as postnominals than adjectives, which almost never do. Participles are found as postnominals in English expressions such as "a house divided", or "sight unseen", but this is archaic and usually used for effect or emphasis.
In business correspondence, the envelope provided, like please see attached the [sth], is something of a "set phrase".
Wikipedia provides a useful article on postpositive adjectives.
In addition, there is a useful Community Wiki answer on the subject of postpositive participles here.
There's no difference in meaning between these sentences, but the former is more idiomatic and common, according to Google Ngram.
