Frist - see my blog on just how it Writes to tape.
Now with that understanding your question about jobs wanting a tape drive.
You have different options on how to set up the use of your tape drives.
When you setup a Storage Unit, you tell it how many tape drives to use at once.
For example we are going to say 2 drives.
You also say how many Streams can go to the tape drive at once.
For example we are going to say 10 streams.
So far we have 2 tape drives with 10 streams per drive - so right now we can have 20 active jobs at once.
10 jobs streaming to driveA and 10 jobs streaming to driveB
Now a job streaming to driveA completes. A job that is queued will now go active and stream to driveA so it has 10 streams again.