There isn't a link. That's why they're called 'transient' reports; the data in these cases is an in-memory collection that is constructed at runtime. The various bits of data could come from the underlying database, or from data that a user has typed into a form that's not going to get persisted, or from a calculation, or (more usually) from some mixture of the above. There's some mapping in the transient report catalogue which explains how a given property of an object in the transient data collection relates to a field in the report designer, but aside from reverse-engineering the code that populates the in-memory collection there's no way of reliably discerning its exact provenance - let alone altering it.

In the case of Stock_Label_5559, this is used by a print method which accepts a data collection that can actually be populated in a variety of different ways, depending on where and when within Sage 200 the labels are being produced. For example, it can be used to print receipt/return items, traceable items, bin items and so on.

To that end, there is little utility in simply attempting to introduce a new field to the transient catalogue, as there's no way to tell Sage 200 how to provide the data for that field - outside of writing a custom label-printing mechanism.

However, depending on the report, it is sometimes possible to link transient data to database tables.

In the stock labels report using transient table Stock_Label_5559, you can link it to StockItems using Code in Report / Joins:

and then add fields from StockItems to the label.

This seems to work but I have no idea how!

Similarly, you could get this report to print multiple labels per item using the technique here

Query has already been answered by colleagues above.