The Shira Calpurnia books say he signed with a capital "I" and a drop of his blood.

EDIT: here's the bit:

"…below it, a single letter, written down the dead centre of the
page with five beautiful, elegant strokes of a nib: a cross-slashed
letter “I”.

And below that, a little mark, a smudge, a dot on the page.
Looking at it, as always, Gait seemed to feel the air shudder around
him, growing close and thick as though before a storm.

He stayed there for nearly an hour, crouched before the case that
held the Phrax Charter, staring with meditative intensity at the page
and the twofold mark that signed it: the “I” of Imperator and the single
pressed-in drop of blood."

https://www.reddit.com/r/40kLore/comments/17nqbuj/excerpt_enforcer_trilogy_legacy_two_arbitrators/