The syllabus year refers to the year in which the candidate is taking the examination.
The question papers are set using the syllabus for the relevant year, for example the Cambridge International AS Level papers for 2018 are based on the 2018 syllabus; the Cambridge International A level papers for 2018 are also based on the 2018 syllabus. The syllabus clearly states the year in which the examination will take place. So, if the candidate is sitting an examination in 2018, then the 2018 syllabus is being tested.
If a candidate sits a Cambridge International AS Level in 2018 then the 2018 syllabus would apply, and if the candidate subsequently sits the Cambridge International A Level in 2019, then the 2019 syllabus should be used. schools where candidates are entered for the Cambridge International A level at the end of a two year course would need to use the syllabus which applies to the year in which all the papers were being taken i.e. candidates being examined for both Cambridge International AS and A Level in 2018 should use the 2018 syllabus.