Are all inline to fix either grammar I noticed or to add in extra details. I forgot I had to rush to an appointment earlier...
- There has not really been a Warmaster since Horus
- After the Heresy Guilliman declared himself Lord Commander
- These two terms are fairly sononymize; Lord Commander has oversight to a degree but Warmaster did not.
- Lord Commander was held by a high Lord of Terra after Guilliman was wounded for a time, but then abandoned.
- After Guilliman's "resurrection" he has reclaimed the title to again stabilize the Imperium
You are correct in a way. Horus was the Warmaster after the Emperor returned to Terra to focus on his secret work. After the Heresy Guilliman claimed the title of Lord Commander to stabilize the Imperium.
Since Guilliman was wounded, both positions have been used. Typically a High Lord of Terra filled the position of Lord Commander, though it was abandoned sometime later. Warmaster has been used in one way or another continuously since the Heresy. When there is a significant threat that requires the resources of multiple sectors one individual is charged the Warmaster by the High Lords of Terra. Sometimes an alternative title is used (given the bad juju of Warmaster) but this individual has complete autonomy to direct and command as he sees fit in the given conflict. After resolution of the conflict the title is no longer active.
Now that Roberto Girlyman has essentially been resurrected, and after both seeing the state of the Imperium and (possibly) consulting with his father in the Imperial Palace, he has declared himself Imperial Regent and Lord Commander (aka the voice of the Emperor and commander of Imperial forces). Notice that he has given himself more power than he had previously as just the Lord Commander.
While neither Warmaster or Lord Commander held the weight they had for mankind since the Heresy like they had for the Primarchs, with a Primarch in the position they both hold a great deal more charge. Until we see the full extent that Guilliman plans to impose himself on the Imperium when the new rulebook comes out we will not know exactly how much power he has, but it will likely be lessened by Guilliman himself. He has always been one to set limits (creating the chapters for example).
Regardless, given the positions being held by a Primarch, the two terms are essentially synonyms. As Warmaster Horus was number two to the Allfather/Emperor, and pretty much coordinated all operations of the military forces without oversight from the Council of Terra (High Lords prior to the Heresy) or anyone save the his father. As Lord Commander Guilliman answered to the High Lords as a member of the governing body. While the High Lords were all equal (as equal as one could be to a Primarch) Guilliman could do as he pleased, though I suspect there was more information disclosure and possibly even discussions on what should happen. I am reading Primarch book one right now and he seems to like having war councils...
Still the two positions were essentially the same...In either situation the Primarch has full command of forces to ensure an overall strategy is being followed and that overall plan not at odds with numerous other strategies by numerous other commanders.