This is the second-most asked question on the forums. It's one of the most common questions we get. It's so common, there's a post about it on the Adepters Code Archive.

From the article: Those Pesky PIs

Problem

On save, Arbortext Editor adds processing instructions, like <?Pub  Caret?>. How do I remove processing instructions (PIs) ?

Solution

Turning off the PIs is easy. You change an advanced setting in the preferences:

set writepi=off

It's documented in help topic #1587.