This is from the Help of Forms [64 Bit] Version 188.8.131.52.0 (Production):
Writes data in the form to the database, but does not perform a database commit. Oracle Forms first validates the form. If there are changes to post to the database, for each block in the form Oracle Forms writes deletes, inserts, and updates to the database.
Any data that you post to the database is committed to the database by the next COMMIT_FORM that executes during the current Runform session. Alternatively, this data can be rolled back by the next CLEAR_FORM.
As far as I know (and we have Forms 6i modules that use POST, which we successfully put on Forms 12c a few years ago), the POST command did not change.
But now there is a difference between COMMIT and COMMIT_FORM, ROLLBACK and CLEAR_FORM.