Yes, but this quantitative reference can be the quantitative reference of other processes, not only of the one you want to link to. In my case, the process I want to use as "input" of another process is "diammonium phosphate production | phosphate fertiliser, as P2O5 | Cutoff, S" (Ecoinvent 3.4) and its quantitative reference is "phosphate fertiliser, as P2O5", which is produced by more than 150 processes.
I think I misunderstood something here but I'd appreciate if you could show me what I got wrong.