How to create Column Identifiers in Google Sheets?

I'm confused by Google's documentation where it states:

Columns are referenced by the identifiers (not by labels). For example, in a Google Spreadsheet, column identifiers are the one or two character column letter (A, B, C, ...).

Then directly proceeds to show some examples...

select * 

select dept, salary

Neither of which appear to be column identifiers as defined.

It further goes on to to define Identifiers

Identifiers (or IDs) are text strings that identify columns.

Which seems contradictory unless they are making a distinction between default identifiers and custom identifiers?

So, how does one create an identifier?