Linda,

When backing up to tape you have to prioritize what data goes on what tape
when considering retention.
Unlike disk, tape is sequential so the only way to keep some data whilst
purging the rest is to copy the required data onto a new tape.

For this reason most backup software adopt the concept of pools. When a
backup strategy is put in place a candidate pool is selected where the pool
reflects the retention requirements.

So for instance in our business we have a daily pool (with a 7 day
retention), a weekly pool (with a 35 day retention), a monthly pool (with a
12 month retention) (this cycle regime is know and Grandfather, Father, Son
http://en.wikipedia.org/wiki/Off-site_Data_Protection#Backup_strategies) and
a permanent pool.

We ensure that the permanent pool is written in tar format
(http://en.wikipedia.org/wiki/Tape_archive) as this format is an open
standard and is supported by most platforms. We use our permanent pool for
source code backups where we start a tape with a full backup and then append
incremental changes on a daily basis. When this tape fills (about every 12
months) we send it offsite to permanent archive.

Our daily, weekly and monthly tapes are sent offsite each morning and do not
return until they expire off-site.

Hope this helps.

Gerard 

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