What does Par1 mean in Excel?

Good evening. I was playing around with LAMBDA and LET, and ran into an unexpected behavior in regards to my variable names. I was being lazy, and attempted to name my first parameter of the lambda "par1", short for parameter 1. It highlighted blue, and when I finished my lambda, I got "you've entered too few arguments for this function", implying par1 didn't count as a variable name. So, par with a number must mean something, but I can't find anything in help, autofill, or a brief search online. To demonstrate what I am talking about:

=LAMBDA(a,LET(x,a,x))("billy")

outputs billy, but

=LAMBDA(par1,LET(x,par1,x))("billy")

gives me the error. Interestingly, just using "par" will work as the variable name. Can anyone explain par1 to me?