Located variables must be of type EBOOL in Unity Pro. Defining located variables as BOOL will cause this error when building the project.
BOOL (BOOLEAN) and EBOOL (Extended BOOLEAN) datatypes each occupy one byte in memory.

BOOL types have two states, 0 (False) or 1 (True) .
EBOOL types have three states, 0 (False), 1 (True) and a History state. This third state requires a byte allocation in PLC memory for the EBOOL..

A reference to any singular IO point on a discrete input or output module would require an EBOOL data type. (%I0.1.0 or %I1)  These are located references because they have an address in Unity
A reference to an unlocated variable can use either data type, BOOL or EBOOL. An unlocated variable has no fixed address in Unity.