what you have found is a drug synonym ID. Multum is a little bit complex here and makes groupings of those synonyms based on “function” IDs, where for example ‘59’ is a Generic Product Name and ‘16’ is a Generic Drug Name. Again, it would be interesting to hear which country this is, as Multum has different “cultures” that they deliver to their clients. My guess is that we are talking UK?
In the US delivery (maybe in other cultures, too) in the client delivery package you will find a PDF about Multum’s Drug Names and Identifiers which presumably will be a good read.
In general, the base of everything in Multum is a DNUM (Drug Number) entry, a representation of a concept very close to ingredient level. The next most important entity is the MMDC (Main Multum Drug Code), which is a Generic Product and as such shares some similarity with the Semantic Clinical Drug (but is not a 100% match). DNUMs and MMDCs should be universally valid throughout all cultures (except for those, where the respective drugs are not available).
The data delivery is normally packaged in MS Access style database format. So, if you are lucky, in your culture you will also find a table called rxn_multum_map. If you search for your code 20280 in the drug synonym colum, you will find that it maps to the MMDC code 3826 and to the RxNorm RXCUI 198440. If you do not have this table, you might want to request this information from Multum to be added to your culture / delivery.
Please let us know, if you had any success.