This post is part of Getting On The Air: Modes, Operations, and Making Contacts.
New hams getting into digital modes hit a wall of identifiers fast, and it is genuinely confusing which one does what. Here is the breakdown.
Your Callsign
Assigned by the FCC when you get licensed, this is your legal identity on the air, required on every mode, every band, every transmission. Everything else on this list is layered on top of it, not a replacement for it. Operating away from home adds its own suffixes, which I cover in Understanding Reciprocal Callsigns.
Your DMR ID
A unique numeric identifier assigned through RadioID.net, required to use any DMR network like Brandmeister or TGIF. Your radio transmits this number, not your callsign, as the actual routing identifier the network uses, though your callsign is tied to it in the database and displayed to other DMR users. Register once, it does not expire, and every DMR radio you own uses the same ID. It is also the first thing you enter when you build your first AnyTone codeplug.
Your APRS SSID
A number from 0 to 15 appended to your callsign (like KZ7K-9) that lets APRS distinguish between multiple stations or device types under the same callsign. Conventions exist for common uses, -9 for mobile, -7 for handheld, and so on, though they are not strictly enforced. If you run more than one APRS-capable device, giving each a distinct SSID keeps them from being confused as the same station on the map. If you are shopping for a handheld with APRS built in, here is what I look for in a ham radio HT.
Why It Is Not More Complicated Than It Looks
Each identifier solves a specific technical problem that your callsign alone cannot, DMR needs a fast numeric lookup for network routing, APRS needs to distinguish devices. None of them replace your callsign as your actual legal identity, they just let specific digital systems do their job more efficiently.