I purchased a Pigma Micron pen for some signature blocks. I checked with the clerk at the store and she said I had the correct pen. After I got home, I noticed it says archival/suited for acid free environments. This sounds like a pen for paper!
Am I correct, or is the clerk?

This is the same kind of pen I have at home. The only thing I have noticed is that is is very thin (tipped) but it works good for signature blocks.

Yes it works fine.

Both of you! For signature quilts or labels I use at least #5, and also use #8

why not a fabric marker?

A fabric marker isn't intended to be permanent .... whereas you want permanent when it's signature blocks

That's the pens I use for signature blocks. Seems to work great.

thats the correct pen, its what I use for a permanent marker on quilts

Yup, you're both right. And it's good that it says it's archival because that means it'll be gentle on your fabric (no harsh chemicals that would eat the cotton).

There are different point sizes. I usually use an .05 for fabric marking.