Your post might be better suited in the IR sub-forum.
I applied IR/DR-only to some programs, IR/DR and DR at some programs, and DR-only (at programs without IR/DR). Summary of my findings:
1. If you get an IR/DR invite and applied for DR at that program, you automatically get a DR interview and can rank both programs separately.
2. If you get an IR/DR invite, but you didn't apply for DR at the program, you probably won't be interviewed for DR (and you can't rank it anyhow without adding it on ERAS). Certain places will have interview days just for the people who applied IR/DR-only (Mt. Sinai in my experience)
3. UCSD rejected me from their DR-research tract, and apparently didn't let anyone who interviewed for IR rank the DR-research (per an email they sent out). I interviewed for IR/DR and DR-only.
Tip: If you apply IR/DR-only at certain places, expect to be asked why you didn't apply for their DR-only program. The reasons I heard on the interview trail were, "Well I'd come here for IR/DR, but I don't want to come here for DR." and "Oh, I must've forgotten to add you on ERAS." Full disclosure: I said both of those things and wish I had been a bit more tactful.