The wiki has been upgraded to MediaWiki 1.43. The wiki's theme is still being adapted to work with the new version, and may appear broken for the time being. --RanAS
External Latch
From SnesLab
The EXTLATCH line connects pin 29 of the S-PPU2 to JPIO7 on the Front Panel Daughterboard, where it is called JPIO7.
According to fullsnes, it is the lightpen signal.
There is also the external latch flag, which is bit 6 of 213Fh.