If the internet is to be believed there are different standard thicknesses of biscuit, as follows.
* 3.5mm. According to the following link the Draper blade is only 3.5mm thick and the reviewer even commented that he had to file normal biscuits to get them to fit.
(Amazon.com product link shortened)
* 3.8mm. The following ad is for a SIP blade that is 3.8mm thick. Is this the real standard blade thickness and their specification is more precise than manufacturers who describe their products as 4mm?
* 4mm seems to be the standard size but it could be a nominal one. The following video shows biscuits being too small for the slots and falling out whereas in other videos I have seen the biscuits are a snug fit even when dry. Could the presenter in this video have the wrong type of biscuit? The relevant section is 12:30 to 14:30.
* Biscuit slots in chipboard.
Chipboard is more compressible than other timber. Would biscuits be as effective in chipboard as they would in other types of board? I recall seeing something about using them to join kitchen worktop but cannot imagine they would be much good in that case if the worktop is chipboard based.
* Biscuit material
I saw a reference to biscuits being made of comressed beech so that they expand once glued and such biscuits have hatching patterns on them where they have been through compression rollers. By contrast I also saw reference to uncompressed birch plywood biscuits that did not have hatching marks. Are the latter designed for MDF or other weaker material? According to the following link, normal biscuits can cause a bulge in MDF. You can see in one comment that it refers to birch plywood biscuits being uncompressed and used in such cases. So it's beech vs birch....