What called as Transcendents are beings who already gone beyond [Growth] and transcend into an almighty being with indefinite lifespan.
They already experienced more than hundreds or thousands of [Growths] (the number depends on their potential). They reached their limits and broken through their limited potential, reaching higher heights.
Their power was obviously tremendous and they are regarded as Gods on MC's current world. For such being they had to depend on themselves if they wanted to get even stronger since they could no longer have any [Growth].
Just a side note that if Goku entered MC's world then obviously he would be regarded as one of the [Transcendent] for his power. So if MC gained the Super Saiyan Blue form he'll automatically became a [Transcendent].
Since Saiyans are different than the people there, they had no real limits since the beginning and reaching the Super Saiyan Blue is what makes them a [Transcendent], a dividing point between normal Saiyans and God-like Saiyans (Saiyans who mastered the Super Saiyan God power).
There's also a Supreme Transcendent that outclass even the most powerful Transcendent. No one know about their existence other than a very select few. Apparently, they only existed on the peak of the 5th Block of Tartarus as the guardian there and (possibly) the never been explored 6th Block. Though it was unknown if this was true or not as no one ever seen a Supreme Transcendent on the outside world.