A tag is treated much like a memory stick formatted to hold either data or birr, but not both. It is locked by a simple code system much like a pin number and probably treated as a bit more disposable (preloaded amounts). A transactor is a more advanced version capable of holding data and birr. The data is typically user specific such as ID and permits. It is locked though more complex means, biometrics (thumbprint, dna, etc) or some other system. Harder to hack into, but the personal data makes you easier to track (p 109)
As far as adding birr, I would suspect the more common way would be to do so at a bank or similar service (think atms). Otherwise, I intend to use specialized tabulas that one or more tags/transactors can hook up to and handle birr transfers. Kinda like mobile debit machines. The tabulas will be capable of holding a certain amount of birr (and possibly a transaction limit) before requiring a bank to update the system.