ASKs and ASSERTs use direction to control which assertions will be accessed during inference. Direction comes in two flavors: forward and backward. An ASSERT with direction :forward will cause inference to be performed at assert time; an ASSERT with direction :backward will cause inference to be deferred until ask time.
A backward GAF is a GAF that will not be involved in forward inference. Forward inference only takes place among rules and GAFs that have a :forward direction. Most interfaces that assert formulas into the KB adhere to the default that implication statements (rules) are backward and GAFs are forward.