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