Why is there is a different UKMEC category for initiation versus continuation for some methods/conditions?
This is explained in the section on Using the UKMEC in Section A of UKMEC 2016.
The initiation (I) and continuation (C) of a method of contraception can sometimes be distinguished and classified differently. The duration of use of a method of contraception prior to the new onset of a medical condition may influence decisions regarding continued use. However, there is no set duration and clinical judgement will be required. For example, the initiation of a progestogen-only pill (POP) is not restricted in a woman with stroke (cerebrovascular accident) as the advantages of using the method generally outweigh the theoretical or proven risks (UKMEC 2). However, if a woman has a stroke (cerebrovascular accident) while using a POP, the continuation of the method will require more expert clinical judgement and/or referral to a specialist contraceptive provider because use of that method would not usually be recommended unless other, more appropriate methods are not available or acceptable (UKMEC 3).