TSAM

tsam is another very common word, used at the end of a noun, for example. It means just, only, merely. It's a way to narrow down a definition so it just covers one topic, or one noun.

In this case, in the commentary it says zhi gnas tsam, zhi gnas was Shamatha, or single-pointed concentration, calm abiding.

So here it means that we are just dealing with the eight chapter dealing with single-pointed concentration.

Just to show other examples where you could see tsam (it's a very common word in commentaries):

sems tsam - mind only, yes, this is the known Tibetan word for the Cittamatra, Mind-Only school.
zla ba tsam - just about a month, zla ba is month, also means moon, so you see the connection to the lunar calendar
tshig tsam - mere words, tshig means word