[Fiahil]

My wife use them for sewing, they disappear pretty well after a rapid ironing ! Much better than chalk or other erasable ink.

[therein]

I remember tailors in Turkey using a thin bar of soap for that back when I was a child.

[Lutzb]

That's tailors'chalk. Looks like a thin bar of soap. https://commons.m.wikimedia.org/wiki/File:Tailor%27s_chalks....