HD and FO : I am not sure, not being an expert on RNP APP and RNAV, but based on my doc ( for ATC) it would seem that the min requirements are what I said, the word " typically " is probably there as you can mitigate the non avail of some component(s) , e.g. . an IRU if you use DME/DME. But not sure.
From another book :
The RNAV system may be based on:
• DME/DME
• DME/IRU
• GNSS/IRU
Also maybe continent specific , in the USA , they use Ground GPS augmentation systems for instance.