Jeff2L refers to any F2L case where neither the corner nor the corresponding edge is in the U layer, and where at least one is in a wrong slot. When there are only such F2L pairs (as it happens all the time to Jeff Chiu , after whom Jeff2L is named), most cubers do for example RUR' to reduce to a normal F2L case.
Below is a complete list of adjacent-slot Jeff2L cases, as well as a few diagonal-slot cases. Since these cases have fairly easy recognition, the easier algorithms may be useful even when there is a normal F2L case.
Corner in wrong slot
Edge in wrong slot
Both corner and edge in wrong slot
Some diagonal-slot Jeff2L