so Garmin did stop using .img for marine chart.
They use a gma at the root with a folder name Charts that containt 2 folders Tiles and a RDPs. In the RDPs got the id of the map and a .SRPD file.
Did i miss a big release note ? Anybody got more info.
thanks

Where is this written? Please provide a link to the studio.
No chartplotter user has ever heard of such a thing. They still use water maps the same way they always have.
Official (or clones) with .gma and .unl files, and without them (cracked).

OP , may mean Navionics+ format.

Sofar - as new chartplotter user i can say - unpatched Garmin Echomap Ultra 2 reads unlocked file, and opens it if .unl file with card CID key is present,
but warns of 90 day trial, then lockout, as it can not verify authenticity with .GMA file.

Last i checked a cracked version of Navionics Boating with charts included - it had Charts catalog in Android - but it did fail after several days of use.
As there is no new VEU050R charts available of Navionics+ or Navionics + Vision for downbload - i can not confirm it for certain to be true on new devices.

Garmin Bluechart G3 uses still old card structure.

\ ----- .id
.GPX (track files)
\GARMIN\ gmapsupp.img
gmapsupp.unl (cardID generated key 25 symbols)
gmapsupp.GMA (Card authenticity control, crypted, signed by Garmin)
Garmindevice.xml (created on first initialization)
On larger cards there will be additional .img files

.GMA file was introduced around 2016 when new Linux-based Garmin ECO system was released, and Garmin dumped old Symbian-based system, as it did was lackluster crypto wise.
.GMA File contains CID (bound to card number), Garmin signature file, and is crypted for new device to be able to decrypt and verify card authenticity.

Newer Navionics+ set's would be interesting to look upon too.

Which should be showing off with Powershell command:
Get-Partition -DriveLetter <your_drive_letter_for_GARMIN_SD-card> | Get-Disk | Select-Object Number, FriendlyName, BusType, SerialNumber

If anyone happens have the package of locked Bluechart G3 and PM me full .rar download with hidden files and their CardID.
I got time at moment to dig and play around as fishing season is concluded more time to fiddle with <other> stuff - already near 0 degrees of Celsius outside ... and only occasional big pikes ... not worth to drive off for yyy km just for day on water, especially with gasoline and diesel prices like now.