the difference is with must include
you get an error if the include file is missing
from QlikView help
The include and must_include variables specify a file that contains text that should be included in the script. The entire script can thus be put in a file. This is a a user-defined variable.
The difference between include and must_include is that include will fail silently if the file is not found during script reload, while must_include will throw an error if the file is not found.
I.e. the construction set Include =filename is not applicable.