Sortkey prefix to end listing BEFORE (not at, if this value occurs it will not be included!). Can only be used with $1sort=sortkey. Overrides $1endhexsortkey.
From Documentation
Sortkey prefix to end listing BEFORE (not at, if this value occurs it will not be included!). Can only be used with $1sort=sortkey. Overrides $1endhexsortkey.