Integral measures in UFL

General
1

Dear all!
I found it while browsing UFL’s related websites：
ufl package — Unified Form Language (UFL) 2021.1.0 documentation (fenics.readthedocs.io)

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What do they all mean except that dx,ds,dS?

2

You’re reading the API documention, which only gives the specific implementation details. You should read the user manual for explanations of design and meaning.
https://fenics.readthedocs.io/projects/ufl/en/latest/manual/form_language.html#forms-and-integrals