In theory you can complete most of the units in any order, but in practice, they are listed in the order they are for good reasons.  PBKT has to be completed before CJBS because they are consecutive units.  Likewise you cannot sit the CJBS exam until you have passed PBKT.

 

Most students like to complete the above two units first because they are the largest and most relevant to their current work and/or getting a job.  The above two units are also the underpinning knowledge that all of your future accountancy study will be built on so it's best to start at the beginning.  You can sit computerised accounts first, but not having the benefit of the knowledge from PBKT and CJBS would make passing computerised accounts far more difficult.