Question about children registered as a British citizen

[Directive/2004/38/EC]

A child can registered as a british citizens (using form MN1) in several cases.

(1) when they are born in the UK and then the parent gets PR
(2) when they are born outside of the UK to a British citizen by decent (if done within one year of birth)
(3) when they are adopted abroad by a British citizen

It is clear that (1) is citizenship "other than by decent".
What are (2) and (3)?

[JAJ]

2. If you mean under section 3(2), it's by descent. [note that there is also section 3(5) registration available in some cases].

3. Registration is under section 3(1) so as long as the child's natural parents are not British citizens, it's citizenship otherwise than by descent.