TTL is total track length: how tall the lens is from the front vertex to the image plane, more or less the stack you must hide in the housing. Optical BFL is where the image wants to sit behind the last glass. Mechanical BFL is where the barrel and the holder think the seat is. Those two back focals are related and not identical. Mixing them up is how a "it screws in" lens sits in a permanent soft focus.
In one sentence. TTL is the height of the glass. BFL is whether that glass can come to a focus on your die in your holder.
Why modules go soft after a "drop-in"
Someone swaps an M12 for a slightly faster M12. The thread matches. The image circle almost matches. The optical BFL is 0.4 mm shorter. The holder was not made for that. You get a camera that is sharp at 40 cm if you unscrew the barrel half a turn and never was at infinity. That is not a lens defect. That is a stack defect.
The bookshelf test
A book that is 22 cm tall does not fit a 20 cm shelf even if the width is right. TTL is the height. BFL is whether the text on the spine still sits where your eye expects when the shelf depth changes. You do not plane the book and call it compatible.
How to filter
On the lens page, TTL, optical BFL, and mechanical BFL are three filters. Use TTL against the housing. Use the BFLs against the holder drawing. If only one BFL is published, ask which one. We do not treat a missing number as zero.