Pretty sure the RR is what was tapped before, not the IRR; that's what I'm hearing the difference is that is significant enough to warrant mention.
That even the RR didn't have the necessary folks for this sort of work, and instead Biden had to tap the IRR, shows how serious the manpower shortage is likely becoming.
Trump's order was a general authorization which enabled call up of any reserve components that the SecDef saw fit.
The Individual Ready Reserve is actually the
largest component of the Ready Reserve, as it is by a substantial margin the most popular choice for service members who have completed their active duty tours but still have time on their eight-year total service commitment. The other Ready Reserve components, both of which are smaller and less popular, are the Selected Reserve and the Inactive National Guard. In practical terms, what these different components mean works like this:
If you choose Individual Ready Reserve when you exit active duty, you have no general service obligations unless activated (only periodic Mustering activities), but also draw no pay and are only entitled to reservist benefits, not veteran benefits.
If you choose Selected Reserve, you are still considered an active soldier/sailor/Marine/airman/spacewhatever, are assigned to a reserve unit, and are required to complete a specific number of drill periods annually. You get full pay and active duty benefits.
If you choose Inactive National Guard, which is only available in the Army, it's basically the same as Individual Ready Reserve, but you also transfer from the Army to the Army National Guard.
The Ready Reserve
program as a whole refers generically to all three options, but there is no such thing as an individual soldier who is just Ready Reserve, you have to choose a specific option.
