I am far from an expert on this subject, but the guy that is. Virtuali, wrote a thread, stickied near the top of this forum about how the nav system and the HUD works. radio system, too. Might want to take a look through it. The Handbook PDF file included with the bird is awful helpful too.
I know that the HSI will display either TACAN or ILS (not both), and NDB (beacon) if you plug it in to the UFC panel. The HSI must have either TCN or or ILS mode selected on the upper left soft-buttons. The direction of the chosen TACAN or ILS transmitter will display as a triangle on the outer part of the compass ring, if it's in range of course. Additionally, the HSI will display all the TACANs, Beacons, Intersections, and/or Airports (any one of the above or all of them), within the HSI's current range setting (Top row of buttons, canter), by selecting them with the soft button on the bottom row, far left button.
The HUD will display the Nav Arrow and ILS indicator lines if one of the add-on carriers or boats added in with AICarriers' frequency is selected. This is kind of a special feature added to Javier's version of the HUD, included in the FSXBA version of the hornet. Each carrier has it's own "frequency" assigned by the HUD. For example, the Nimitz version is 111.00 (TCN 47X). This will also automatically set the Course Line onto the NAV1 selector of the HUD Control Panel. It can be a bit odd watching the course setting for NAV1 change as the boat makes a turn. Pretty handy for a quick reference for lining up on. The ILS indicators show up when behind the boat (within, I think, 10° of BRC, either side).
Unfortunately, the HSI displays none of this information for the carriers put in with AICarriers, like the HUD will. So it's a choice, essentially, between the steering arrow and full ILS, OR the HSI Course Selected line and only the ILS steering line on the HUD. Most of the static add-ons seem to have no glideslope information for some reason.
I hope all my rambling helps a little bit, rather than just made things worse
Pat☺