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
On save, Arbortext Editor adds processing instructions, like <?Pub Caret?>. How do I remove processing instructions (PIs) ?
Turning off the PIs is easy. You change an advanced setting in the preferences:
It's documented in help topic #1587.