There are terms used in this page defined above in the document.
Here is a short description.
If you hold a node key to N (a node) you can form a segment key to N with an 8 bit data byte of your choosing.
Four of those bits are called “lss” (log segment size).
The other 4 bits are the number of ‘initial slots’ in the designated node.
If that count is n then slots 0 thru n–1 of the node are initial slots.
(Such slots define memory. Other slots set rules or are ignored.)