What does the term “Secondary Steelwork” mean?

There is no generally accepted definition for secondary steelwork.

In the first interpretation the definitions given in clauses 3.7 of BS EN 10225 may be used as a reference:

Clause 3.7 Secondary elements
‘elements of minor importance, failure of which would be unlikely to affect the overall integrity of the installation’.

More you can read on BCSA memorandum No 158-12.