You begin claiming CCA on your vehicle the "first year it was used for business purposes". You will claim everything as an "addition" for the first year you began your business or actually began using that vehicle in the business.
For example: If you did not start your business until May 2018, but purchased your vehicle in 2017: you will "add" the vehicle at its Fair Market Value (FMV) in May 2018. The first year of business "the half-year rule" applies so you will only get 15% of the CCA claim, which is then subtracted from the opening balance, leaving you with a "UCC" amount.
FMV vehicle May 2018 - $10,000
30% CCA rate (half year rule applied) $1,500
UCC to carry forward to next year $8,500
The following year your claim will be 30% of the $8,500, and that amount will be reduced leaving you with your UCC, and so on.