Accurate definition of "unique" in naming

Writing a chemical structure, which contains iso-octyl, can't figure out the specific meaning of “difference”. Isn't it possible to be different as long as it is not a linear "positive" alkane? I understand that among the many isomers, only the group with two methyl groups at the end can be called “exclusive”, and I don’t know right or wrong. There is no suitable book at hand to find out, and the few that are said are rather vague, or are not clear.

I used to learn about macromolecules and I didn’t pay much attention to organics. Now I’ve found out that there are many places to use and I’m ready to study. Can we recommend an organic textbook? Check it out. There are too many textbooks for organic chemistry. It’s hard to tell which one is good. .

A1: For straight-chain alkanes, the number of carbon atoms is (positive) a certain number of alkanes, where the word "positive" can be omitted.

For branched alkanes, if the molecule contains a (CH3)2CH-structured alkanes called iso-methane

If the molecule contains a (CH3)3C-structured alkane called neoalkane

A2: However, industrial isooctane is a special case that does not comply with the above-mentioned rules.

Its structure is: 2,2,4-trimethylpentane