Scratching my head to work out what all that action complexity and enhanced ring of UJ is for. Is there a lower part of that large ring that closes and shades the venting on some notes such as a long Bb?
Usually that is what is referred to as the "forked Eb/Bb" option. It permits you to trill from clarion Ab to Bb (or chalumeau Db to Eb) by playing the Ab normally and trilling to Bb by moving only the middle finger of the left hand. It is useful in any circumstance where you need to move quickly and cleanly between these two notes and need a good clear sound on the Bb. On a standard Boehm clarinet, you trill the Ab to Bb by moving the 1st finger of the left hand. The Bb is very poor sounding but adequate for a trill if properly supported. However in circumstances where you need a nice clear Bb, you could not use this Bb fingering.