When it comes to planning the domain names can I use the following:
ax.d365ffo.onprem.mydomainname.local
sf.d365ffo.onprem.mydomainname.local
or must it end with .com ?

When it comes to planning the domain names can I use the following:
ax.d365ffo.onprem.mydomainname.local
sf.d365ffo.onprem.mydomainname.local
or must it end with .com ?
There is no mandatory limitation to the URL format. You can use whatever you want to plan the domain for on-prem. Below is just some sample