Depends on the program. The funder sets the rule in each program's guidelines, so check the program a PI is applying to.
What applicants may do
What the funder asks of applicants
What is off limits
Rules for reviewers
American Association for the Advancement of Science has published no rules for reviewers.
Where this comes from
The funder's own words, from its own pages.
Artificial Intelligence Agreement: Applicants must affirm that text generated from artificial intelligence (AI), machine learning, or similar algorithmic tools have not been used to generate the submission nor its accompanying figures, images, or graphics.
Limited editing with tools such as Grammarly are acceptable but must be declared in the section included at the end of the application.
(Optional) Artificial Intelligence Declaration.
Application Tips - Mass Media Science & Engineering Fellowship (opens in a new tab)
Text generated from artificial intelligence (AI), machine learning, or similar algorithmic tools cannot be used to generate the submission nor its accompanying figures, images, or graphics.
Limited editing, with tools such as Grammarly, is acceptable but must be declared.
See American Association for the Advancement of Science's 3 open programs
This is a reading of what the funder has published, and it is not legal advice. A solicitation can set its own rule, so read the funding announcement before a PI applies.