"Loose Yoshi" flag, and slot (index to $7E:009E and similar) of the current Yoshi, plus 1. Gets its value from $18DF. If set, Yoshis spawned from an egg will become a 1-up instead.
For finding Yoshi's slot within custom sprite code, it is preferable to use this address, since $18DF is reset every frame before being set by Yoshi again.