As Imeo told me before and now confirms it on the forum: no need to take it off, you can use all! 4 options
I respectfully disagree Emonty and this is only serving to confuse and frustrate the consumer.
The user(s) original question(s) were two fold, to which there is a very clear, concise answer:
First question: Can i screw all four atomizer types (801, 510, 901 and 901 cartomizers) onto the unit without any type of adapters?
Answer: No. The 510 and 801 screw on with no type of adapter needed. The 901 and 901 cartomizers need a small brass adapter (also known as a "part", "thing", "threads") in order to adapt its threads to the units threads.
Why the need for an adapter? Because the parent Unit's two thread types are 9mm (for the 801 atomizer) and 7mm (for the 510 atomizer). The 901 atomizer has a different 8mm thread and thus will not screw on without an adapter that adapts its 8mm thread to the thread type of the parent unit.
Second Question: Can i just leave this 901 adapter on the unit and still put on my 801 and 510 atomizer.
Answer. No. It must be removed in order to utilize the 510 and 801 atomizer. As Imeo indicated, it will probably just come right off with your 901 atomizer. Should it unscrew from the atomizer instead and stay on the unit, you'll need to remove it.
I hope this clears up any confusion.