The Brass Overlays in GPO are probably the most incorrectly used "instruments" in GPO. The word "instruments" is in quotes, because the Overlays are unique in GPO in that they're the only patches which aren't actually instruments, but instead, are - well, they're sounds.
The theory behind these Overlays is to add that brassy, metallic ringing sound characteristic of brass instruments when played at forte levels. We all know that brass samples can sound a bit dull and unconvincing, and so to help with that issue, the programmers for GPO added these very useful Overlay sounds.
The way they're sometimes used incorrectly is when they've been inserted and used as if they're actual instruments. Once in awhile I'll see the instrument list for a project, and a brass Overlay will be listed without the associated instrument itself being used. The effect is strange and unnatural.
Even if you insert an Overlay into ARIA and use it as an additional layer, but have it play full bore through an entire piece, the results are akin to having someone shout at the same volume non-stop for several minutes, instead of speaking with expression, or singing. Yikes.
Instead of just describing a track properly using an Overlay, let me go through the steps of using these patches. The effect achieved will be clear from this mini-tute, and there's an MP3 example posted below. I'll use a French Horn as an example, and as usual, Sonar is my software of reference:
1-Record your Horn's line in your project file.
2-After editing the MIDI track to your satisfaction, duplicate that track into a new track which is connected to one of the Horn Overlay sounds in ARIA. In Sonar, you can highlight the original track, then while holding Shift+Ctrl, drag the data down to a new track. That insures that the MIDI clip(s) is in exactly the same position. Alternatively, you can Copy and Paste in the standard way.
3-In the Piano Roll View, have the Overlay track open. Use the Eraser Tool and sweep through the CC1 (or CC11) Controller Pane, erasing all that volume data.
4-Re-wind the project and arm the Overlay track for recording.
5-Either have all the tracks set to play, or solo the Horn and Overlay tracks if you need to concentrate on those alone.
6-Record the volume performance for the Overlay track, using your Mod Wheel, or whatever device you use for recording either CC1 or CC11.
MOST OF THE TIME, you'll have the Overlay either down to zero, or at least under 30. You'll want to swoop up on the loudest notes of the Horn's line, making the instrument "ring out" the way horns do in concert.
Here's a screenshot and MP3 example I posted in a mini-tute for someone last year:
That's all there is to it - the purpose and use of the Overlays. And note there are several flavors of Overlays for all of the brass in GPO, namely "f" "f AG (aggressive) and "ff."