What is a semicolon in a makefile prerequisites list?

Question

In one of the early sections of the GNU make manual, Section 3.7, there is an outline of a makefile recipe

immediate : immediate ; deferred
        deferred

involving a semicolon in the prerequisites list. So a valid recipe could be

output.o : output.c header1.h header2.h; header3.h
        gcc output.c -o output.o

What is the point of the semicolon? Since the aforementioned section talks about immediate and deferred variable expansion, I am guessing that the part up to the semicolon is expanded immediately and the part of the semicolon is expanded only once the target is executed. Can anybody confirm?

I have found similar questions on SO but those present special cases and none of the their accepted answers seem to get the point.

Answer 1

As explained in 4.2 Rule Syntax:

In general, a rule looks like this:

targets : prerequisites
        recipe
        …

or like this:

targets : prerequisites ; recipe
        recipe
        …

[...]

The recipe lines start with a tab character (or the first character in the value of the .RECIPEPREFIX variable; see Special Variables). The first recipe line may appear on the line after the prerequisites, with a tab character, or may appear on the same line, with a semicolon. Either way, the effect is the same.

(Emphasis mine.)

Your example is equivalent to

output.o : output.c header1.h header2.h
        header3.h
        gcc output.c -o output.o

Answer 2

Semicolon simply allows to write the first line of the recipe on the same line as the prerequisites list.

This way (given that your recipes fit a single line) you can write a Makefile w/o any evil tabs. There's no much use in it otherwise.