I don't know of a standard list of codes other than the presets in the drop down, but the recommendation I have always heard is keep it simple.
The help file shows only a few examples of unique (non-standard) button codes, which happens to be for pipe. (see the topic: Design Line Button Codes)
The better place for examples would be to look at the services that come with the software. (such as: ADSK - HVAC: Supply Air)
When you have problems with button codes not filling in, it is sometimes useful to use the Fill Report dialog.
When design line is active, and right after you fill in a line, go to the View menu and choose Troubleshoot.
Look for lines that have a Failed status, click on it, and then you can at least see what mappings it tried to use.
It will sometimes give an explanation as to why it failed.
For your particular issue with the rectangular tap, at a bare minimum you will need a mapping that reads something like this:
- Button Code = RedIlTee
- Alternate Codes = Straight, Shoe
Your straight button needs to have the Button Code of: Straight
Your tap button needs to have the Button Code of: Shoe
You don't have to use the code Straight or the code Shoe, the important part is that the code on the button matches the code used in the mapping.
The Autodesk service I mentioned near the beginning uses the button codes above, but uses some more advanced mappings:
- Straight, RedInLine, Shoe
- Straight, RedInLine, Shoe, ShapeChange
I hope that helps.