IB subject selection is made once, at the start of the two years, and it shapes which university programmes remain open. This makes it one of the highest leverage decisions in the diploma.
The six group structure
One subject is taken from each of studies in language and literature, language acquisition, individuals and societies, sciences, and mathematics. The sixth comes from the arts or is a second subject from one of the earlier groups.
Three are taken at Higher Level. The structure guarantees breadth, and the remaining freedom is in which subject fills each group and which three go at Higher Level.
The intended field decides
Medicine expects chemistry, usually at Higher Level, and normally biology. Engineering expects Higher Level mathematics and physics. Economics programmes expect mathematics, and the more quantitative ones name Analysis and Approaches.
The requirements are published by each university. They are read before the selection, because after the selection they become constraints rather than choices.
Mathematics is a decision of its own
The choice between Analysis and Approaches and Applications and Interpretation, and between Standard and Higher Level, closes or keeps open more degree programmes than any other single subject choice.
It is treated separately under Math AA and AI, because the reasoning is different from the other five groups.
First language and acquired language
The language and literature group is normally taken in the student's strongest language, and the language acquisition group in a second one. For a student in Turkey this often means Turkish in the first group and English in the second, or the reverse where schooling has been in English.
The combination is worth checking against the destination: some universities in Turkey and abroad ask for evidence of language proficiency separately, and an IB language subject does not always satisfy it.
The window for changes
Changes are generally possible in the first weeks and become difficult afterwards, since the two years of content build continuously. Schools publish their own deadline and it is earlier than the official one.
The order of the decision
Destination, then degree, then the subjects that degree requires, then the level for each. Starting from which subject is liked produces gaps that appear only in the application year, when they cannot be fixed.