If the TPR molblock is named "Protein_chain_XXX" then we assume that XXX is describing the chain of a protein (in the sense of the PDB chainID) and set the topology attribute chainID to "XXX". In all other cases, the chainID remains the full molblock name. The segID is never changed.
With @pgbarletta's PR https://github.com/MDAnalysis/mdanalysis/pull/4281 merged (for issue https://github.com/MDAnalysis/mdanalysis/issues/3994), the TPRParser is now setting chainID for the topology.
This should be documented in the UserGuide, too. For example:
Long form description