Many values in a template exist only once the stack runs: a generated bucket name, a table's ARN, the region. Intrinsic functions compute them at deploy time, and pseudo parameters such as AWS 24 ::Region supply facts about the stack itself. Every function has a full name (Fn::Sub: in YAML, {"Fn::Sub": ...} in JSON) and a YAML short tag (!Sub). They work in resource properties, outputs, metadata and update-policy attributes and conditions, never in parameter definitions or logical IDs. Fn::ImportValue and the newer Fn::GetStackOutput, which read other stacks, wait for Cross-Stack and Nested, and Fn::Transform for Macros and Extensions.
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Intrinsic Functions
Intrinsic Functions and Pseudo Parameters