What is the difference between PER_ALL_ASSIGNMENTS_M and PER_ALL_ASSIGNMENTS_F in Oracle Fusion

PER_ALL_ASSIGNMENTS_M is the core table for assignments.
PER_ALL_ASSIGNMENTS_F is a view which stores effective latest change only.

The definition of the PER_ALL_ASSIGNMENTS_F view:

SELECT *

  FROM per_all_assignments_m

WHERE  EFFECTIVE_LATEST_CHANGE = 'Y';

If you want all the data then use the base table : PER_ALL_ASSIGNMENTS_M

If you want the effective latest change only, please use : PER_ALL_ASSIGNMENTS_F