Digi+ refers to the PWM IC instead of analog. MOSFETs, Drivers, Inductors(Chokes), and capacitors
are all analog in a sense, they deal with Voltage, and continuous voltage. Digital refers to 1s and 0s.
basically the PWM controls the other components, it is given a reference voltage and an error voltage, and it compares those two and depending on the difference it tells the drivers to increase or decrease the output to reflect the difference.
Digital PWMs take the error signal, which is a voltage, and use an ADC to convert it to 1s and 0s, then compared to the reference, and then its analyzed by a PID algorithm, and then duty cycle is changed.
Analog PWMs take the error signal and amplify and then compare it against the reference voltage through a comparator, then the duty cycle is changed.
Digital has more control over that difference and the amount of change on the duty cycle. Digital is also more precise in that it can pre determine how long to change the duty cycle, while analog will wait for a change in error signal before changing.
Analog is faster and digital is slower, so that waiting for error to change isn't necessarily a bad thing when you are working 5 times as fast. But now Digital PWm tech has come a long way over the past 5 years, and they are just as fast as analog, so the choice is become more clear as asus and GB adopt digital, which they both have. Except GB just adopted it. ASRock adpots everything ASUS adopts because they are really owned by asus.
Anyways Digital also has much more end user control over the phases and how often they fire and such.
Digital is more dense, its implementation is better, it has NVM onboard that can do LLC and different modes like gaming and overclocking, analog needs add on ICs to do that.
Intel spec demands a change to digital, as they are changing their signals like SVID to digital, while some digital PWMs, well all use the digital DVID signal and convert it to analog and then back to digital, its weird i know.
In the end the difference is very small, the way the PWm signal is changed and outputted is the same. You can basically just replace the PWM chip and call it digital VRM. but don't ever say digital VRM, as there is no such thing. its a digital PWM. Or a VRm based on digital PWm technology.