Reuters Dataset Labels description
Apparently the paper introducing the dataset mentions a list of &quot;online appendices&quot; (table 1 p 363) which seems to contain details about the categories. However I wasn't able to find these appendices in the additional material.

[edit] Since it appears that there is no existing source for the original list of topics, I think your best bet is to rely on an acronym dictionary. Based on Acronym Finder, given that the data is mostly about economic news, I think the most likely meaning for the three topics you mention are:

DLR: dollar
GDP: Gross Domestic Product
BOP: Balance Of Payments

You could check whether these are correct by looking at a few documents labelled with these topics in the dataset.