DC Comics is publishing company founded in 1935.
From the beginning, Super Comics has had a relationship with DC, though the two have always been independent of each other.
Starting with its first issue, Super Comics has published golden age Superman tales as one of its features. In the early 80s, Super Comics had guest appearances from Dial "H" for Hero and Batman, and Powerkid # 3 featured an appearance from Superman, that was in the Earth-1 canon.
In 1985, the Super Comics line of title participated in DC's 12 issue crossover series titled Crisis on Infinite Earths, even spawning it's own series, the Crisis Within.
Following that series, and DC's reboot, the Super Comics titles were now considered to be part of the DC Universe, though the two companies stayed separate logistically. However, in 1993, Super Comics decided to remove all elements of the DC Universe from its own continuity, without retroactively removing it, by revealing that the Super Universe was a Hypertimeline.
In 1994, Super Comics launched Super Comics DCU, in which new versions of classic Super Comics characters were featured in titles taking place directly in the DCU, but the titles all ended after just six months.
In 2000, several Super Comics characters actually moved to the DCU, and lived there for a year.
From 2003 to 2007, Powerman's adventures had him travelling through time, space, and alternate realities, meeting many famous characters from other properties, including DC characters from current and older continuities. It was always left ambiguous which reality he was in when meeting these characters. For example, when he met the golden age Superman, he may have been in the Super Comics Universe, the Wold Newton Universe, or Earth-2.
With the 2008 Super Explosion, references have been made acknowledging the DC characters having counterparts in the Super Universe.