A, AS, and A2 levels are only referred to Cambridge International AS & A Level syllabuses. For IGCSE syllabuses, please refer: Core & Extended Syllabus
The article below contains information provided by the Cambridge Assessment International Education website. You may read it here: Cambridge International AS & A Levels Qualification
There are three ways to approach Cambridge Assessment International Education A Level examinations:
Take the Cambridge International AS Level only. The syllabus content is half of a Cambridge International A Level.
Take a 'staged' assessment route — Cambridge International AS Level in one examination series and complete the final Cambridge International A2 Level at a following series, usually in a span of 2 years.
Take all papers of the Cambridge International A Level course in one examination series
A simple thing to keep in mind is:
AS + A2 = complete A Level
A2 Level papers are generally harder than AS Level as they required more analysis and critical thinking. As a result, A Level provides the grade scale from A* to E, while AS Level provides from A to E.
In Step 2: Subject Selection of the exam registration, AS Level papers are labelled with AS Level, A2 Level papers with A2 Level, and the complete A Level papers are not labelled.
If you're unsure of the level you should choose, please consult your teacher(s).