I'm revising for a German exam I have coming up, and I came across the sentence
Aufgrund seiner guten Leistungen wählte ihn seine Produktionsgruppe zum Gruppensprecher
which I understand to mean 'Due to his good achievements, his production group chose him as group leader.'
Until now, I've only ever seen „als“ used in situations like this. For example
Sie wählten ihn als Wächter für sie aus
which I understand to mean 'They chose him as the guard for her.'
I've looked online a bit, but so far I haven't found any explanation. When is „als“ used, and when „zu“? Does it depend on context?