Improvisation is the most important of all musical genres in Persian classical music, principally carried out in free meter. Being instrumental or vocal, the genre is usualyy known as avaz (not to be mixed up with ''avaz'' as related to ''dastgah'' as mode). In avaz, instrumentalist or vocalist shows his utmost technical dexterity. A brilliant musician is expected to master all dastgahs and avazs and be able to improvise in them for rather long period of time. The ability to develop gushes and link them together in a proper way, in addition to technical mastery, is a prerequisitw to the art of improvising.
The most sophisticated type of improvisation is called morakkab-navazi or morakkab-khani with modulations between dastgahs. It demands great mastery and artisanship. Instrumentalist and vocalist may also improvise certain sections in fixed meter. In this case , either chaharmezrab (It will be explained) or any undefined fix-metered piece is played and ''zarbi'', a fixed metered vocal piece, is performed in which melodies are applied in an improvisatory manner to poetry in syllabic and metric styles. By the way, both instrumentalists and vocalists could use fix-metered sections of radif and begin to develop them.
Other musical genres are fix-metered and for the most part instrumental.
Source: An Introduction to Iraninan music booklet from Mahoor Institute of Culture and Art