The USE and EXEC keywords let you use other program slots.
USE will compile the slot, which lets you use COMMON functions, access all of its functions with CALL "slot:name", and access variables (but not arrays) with VAR("slot:name").
EXEC compiles and executes the code, so it lets you do everything USE can do, along with running the code.
Both of these will prevent you from using PRG commands on the slot.
Both commands have two syntaxes. You can either give the slot number (0-3) or a filename "PRGslot:filename" which will be loaded into that slot without any dialog.
For example, instead of:LOAD "PRG1:LIBRARY",FALSE
USE 1you can do: