A member may earn either a single drill or a multiple drill while in IDT drill status, but only one drill per day while serving in AD or ADT status...
Speak English, man!
The 2-drill per day criterion has an interesting history.
Reserve or militia drills were originally accounted during the early years of our country by dedicating one day per week for drilling or rehearsing maneuvers by the reserve/militia. Christmas and July 4th were reserved as formal holidays and excuse from any formally paid drilling.
From those 52 weeks minus 2 holiday weeks came the 50 drills a year criterion for a satisfactory year for longevity and retirement purposes.
It was also during this period that the 4 hour criterion was adopted as a minimim time period fro a 'drill'. in that many reservists/militiamen had to journey by horseback some distance to get to a drilling assembly area.
Circa WWI, tactics and demographics had changed considerably, and it was recognized that more training could be performed by field maneuvers over a several day period. The 50 drills/yr amd 4 hr min drill period remained as regulatory constraints for federal pay/ Within those constraints, and larger regions to recruit servicemen, came arguments for a weeken drill per month en lieu of a one day drill per week. More training value was actually obtained and less percentage of the serviceman's time wasted on travel.
Accordingly, the weekend drill was adopted and the 4 hr drill period remained.
What varies today is travel pay. For many years, even for drills in remote locations, travel from home to the assembly area was the serviceman's burden, while travel while under orders was a government expense. The grey area comes in discerning whether the servicemen is obligated to remain faithful to all regulations 24-7 even in civilian status as is customary, or if his obligation only commences upon assembly at the reserve center.
I know of many situations where servicemen paid their entire paychecks and more simply to travel to drills without reimbursment for much of their career, while 20 years later, their juniors also receive milage and perdiem venturing to the drill site. Many of these situations vary from command to command and interpretation of the regs over time.
On top of these requirements came the 2-week active duty periods which only changed in formal nature about 15 years ago. IDT prior to this wasn't recognized, rather a rule that while in uniform the servicemember had the same obligations and privledges as a regular.
Over the past 20 years there also had been considerations to migrate from a weekend drill plus 2 week active duty routine to a 6 week contiguous active duty period, because in more modern maneuver warfare environments and higher tech battlefields, some argue more training and unit performance is enhanced by a longer active duty environment. 3 days without sleep doesn;t compare to a 6 wk intensive training environment for skill retention.
The arguments today might be even more persuasive considering the pressure on civilian job markets by extensive overseas deployments by reservists.