Each subsection of dastgah or avaz is called a ''gushe''. Some gushes are free- and some are fix-metered. In addition, a quite number of gushes are only melodically significant and have the capacity to accommodate to different modes, hence their presence in almost all dastgahs and avazes.The remaining ones, those with more importance, have modal significance and create different modal ambience. For example, a certain type of gushe known as Daramad (''introduction'') launches dastgah/avaz and stabilizes the principal mode. Kereshme, Naghme, Baste-Negar, and Majles-Afruz are amongst the first type, and Delkash, Shekaste, Zabol, and Mokhalef amongst the seconde type.
Dastgah-e Mahur comprises of almost 38 gushes of first (Kereshme, Majles-Afruz, Zangule, etc.) and second types. The latter type secures the modal diversity od dastgah. Of these, Daramad appearing first of all, is in Mahur mode.
Some other gushes are dad, Delkash, Hesar, Shekasteh, Araq and Rak; this mean that in dastgah-e Mahur, music modulates to Delkash, Hesar, Shekaste, Araq and Rak.
Reaching at this point and using the aforementioned concepts, radif is again defined as a model repertoire comprising different gushes (between 250 to 300 in quantity) all classified in seven dastgahs and avazs.
To summarize this, we might regard the Persian classical music as a tree deeply rooted in its long history. The main trunk is radif and to be more pricise, is both instrumental and vocal radif. Out of this trunk, seven branches are drived forming seven dastgahs. Some secondary branches from five types of avazes.
each of these twelve branches possesses different gushes, some are in special modes and others not modally defined. Besides, each dastgah or avaz contains a principal mode as is represented in Daramad. During a performance, gushes all return to this principal mode.
The tree of Persian music is thus formed by its old roots, by radif as its trunk, by dastgahs as its main branches, by avazes as its secondary branches and by gushes as its numerous stems. The leaves and fruits are the pieces composed and improvised. These are nurtured by the latex flowing in its veins. To articulate it better, radif is the main source of composed pieces by musicians.
Source: An Introduction to Iraninan music booklet from Mahoor Institute of Culture and Art