Agn files are autogen only, you can't open them directly, but you can edit them in Annotator. If it's a photo scenery autogen, then you can open the BGL file from the corresponding scenery folder, and it will display the autogen from the Texture folder. See the SDK for details of how to use Annotator.
I think I have some better understanding of them now. I was wondering if the .agn files were autogen files and now I know. These files were located in my Texture folder of the KDFW or the Dallas Fort Worth airport scenery by fsdreamteam. Let me explain what I'm trying to do here but I guess I should do this in another thread that would be more appropriate. The thread will be under the name "Editing Ground Textures for Updated Airport Scenery," or something like that.
PS: If you plan to publicly distribute your scenery modifications for use by anyone else, you should NOT edit default FS files, but instead learn how to "Exclude and Replace" default scenery content with custom content, as this would require end users to use your edited *.AGN files.
This can be done using "Exclude" BGLs, and custom photo-real aerial imagery land class BGLs that incorporate Blend Masks when compiled.
These BGLs can be placed in a "active" Area layer above the default or 3rd party BGLs to be 'modified' by your custom BGLs that modify- / substitute for- display of the default scenery.
If you only want to edit the Autogen for a default scenery land class area, you can "Exclude" the default Autogen with custom "Exclude Rectangles" that specifically target Autogen objects.
These "Exclude Rectangle" BGLs can be created and output to a BGLComp XML-type BGL via AFX or ADE etc..
Alternatively, a custom photo-real aerial imagery land class BGL can be used that incorporates a Blend Mask when compiled.
The Blend Mask can control where you will see:
* default land class textures with their associated default Autogen annotations
...and where you will instead see:
* custom photo-real aerial imagery with any associated custom Autogen annotations
...within the Geographic extent of coverage for your project BGL.
NOTE: Custom Autogen annotations that "Replace" the default 'Excluded' Autogen annotations will be placed onto the 'new' custom photo-real aerial imagery land class texture (which can either have a visible aerial image ...or be a purely 'transparent' surface layer with NO visible aerial image if desired).