What is not clear: what a section is, or something else?
If it is a section, that I think I understand (after reading the first posts more properly: what @digipl said, but more elaborate).
Each section is defined by a name prefix, i.e. a sequence of between 0 and XOR_NAME_BITS (currently 256) bits, similar to how IP subnets correspond to IP address prefixes.
Every node gets an address in the Xor space wich is a number, 256 bits long: so between 0 and 2^256 (very big).
Assume for example that the SAFE Network will have 65536 nodes in total with e.g. an average of 16 members in each group.
Then you have approximately 4096 (2 to the 12th power) groups in total, meaning the average section is 12 bits longs. So for example a section could be every Xor address that starts with 12 zeros (if you understand the working of blockchain, this should sound familiar). So in my example about 16 of the 65536 total number of nodes will be member of section ‘000000000000’.
And this section will have a total 2^256 / 2^12 = 2^244 members. Or, in other words, the total amount of numbers between 0 and 2^256 that start with 12 zero’s. And that group of 16 is a subset of these members.
Or maybe the different type of groups is meant: group of elders, group of adults, group of infants, and the ‘total’ group, that contain all these groups?