A. You could use each note in any register (octave) you wanted.
B. You could turn it around (retrograde).
C. You could turn it upside down (inversion).
D. You could turn it around and upside down (retrograde & inversion).
E. You could transpose your sequence of notes so that it could start on another of the 11 available notes, therefore supplying 47 other versions of the tone row that you started with.
F. You could only use part of any of the 48 tone rows if you wished.
Now we all know that this system of composition caught on quite dramatically and is still used to this day of after all these years. Even Stravinsky, who was always usually at loggerheads with Schoenberg (although deep down, they probably had a begrudging mutual respect for each other) was composing in this manner towards the end of his life. However, Schoenberg gave it up as dramatically as he had embraced it. Before he died, he admitted that atonality in music was really a non-starter because it did not adhere to the natural harmonic series (which is what the major key is derived from, which is why listeners feel more at home with tonal music because the material has a 'home' to go to, which is the tonic of the key in which the music is).
When I was at university, it was assumed that all composition students would not necessarily compose using the 12-tone method, but they were expected to be very economical with the use of tonality. This saddened me, because I have always preferred tonal (and modal) music for the reasons I've stated above.
However, now and again, I have attempted to try and see a way around the rules and tried to compose music of this fashion and make it as accessible to the listener. This is my most recent attempt, and, as it happens, my first attempt with a full orchestra.
Suffice to say, I failed miserably.