Question: formal parameters in procedures - why formal?

In the Help section under procedures (or proc) it states:
argseq - the formal parameter names
I just wondered about the use of the word 'formal'.
eg in this simple Maple code
maxim:=proc (a,b,c,d)
end proc:
. . . the formal parameters are a, b, c, d. Is the word 'formal
' being used as "officially recognised" and a=2, b=3 etc. (ie the
order is important or is there
some deeper meaning? I'd appreciate a short example of a proc using
something else besides variable values, or Null.
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