In representation theory terminology: Branching rule vs decomposition?

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In representation theory's terminology, what are the differences between

  • Branching rule?

  • Decomposition?

We can take any spin group as examples such as Spin(3) or Spin(8). Are Branching rule vs decomposition the same thing or different?

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'Decomposition' means that you are given a module and want to write it as a sum of (probably indecomposable) modules, or possibly compute its composition factors. This could be a restricted module, or a tensor product, or anything else.

A branching rule on the other hand is a general description of how to restrict representations to subgroups (or induce from). Usually the group and subgroup are geometrically or combinatorially defined (for example, $S_{n-1}\leq S_n$ or $\mathrm{GL}_{n-1}(q)\leq \mathrm{GL}_n(q)$), and the rule is usually combinatorial in nature. For both of the examples I gave, the ordinary characters and unipotent characters respectively are labelled by partitions, and restriction and Harish-Chandra restriction respectively are given by removing a (removable) box from the partition.