
The main installation folder is the one whose path is recorded in the product's Uninstall information on the target system and is used to calculate the approximate installation size. PRIMARYFOLDER A symbolic variable that defines which folder (alias) is considered to be the "main" installation folder. INSTALLDIR The folder alias of the predefined product installation folder.

This article explains what they are used for.

All predefined InstallMate projects contain the symbolic variables INSTALLDIR and PRIMARYFOLDER.
