While this is of course "inside intel" the OP is posting from USA and the post specifically states
"max gridded altitude " which I don't know of anything like that in the other countries. So I think we're safe to "assume" they meant FAA UAS Facility Maps.
Here's a link to the main FAA page:
www.faa.gov
First paragraph from that page reads:
UAS Facility Maps show the maximum altitudes around airports where the FAA may authorize part 107 UAS operations without additional safety analysis. The maps should be used to inform requests for part 107 airspace authorizations and waivers in controlled airspace.
and here is the disclaimer on that very page:
NOTE: These maps DO NOT authorize operations in these areas at the depicted altitudes – they are for informational purposes ONLY. Operators must still apply to operate in controlled airspace (Class B, C, D, or surface area E) by completing a Part 107 Airspace Authorization or Waiver Application in the FAA DroneZone.