I wouldn't normally just post a link to someone else's work here, but in this case Frank Wong - a colleague of mine at my new company (DGit Systems) has done some terrific work in helping to eliminate the miss-match between the data model used by the TMF's REST based APIs and the TMF's Information Model (SID). I know this was an issue that IBM were also looking to resolve. In the effort to encourage the use of a simple REST interface, the data model used in the TMF's APIs has been greatly simplified from the comprehensive (some might say complex) data model that is the TMF's Information Model (SID). This meant that a CSP who is using the SID internally to connect internal systems needed to map to the simplified API data model to expose those APIs externally - there was no easy one-to-one mapping for that mapping which meant that the one could not simply create a API for an existing business service (eTOM or otherwise) - a lot more custom data modelling work would be required.
This interview with Frank by the TMF illustrates some of the latest work to resolve that miss-match - read it at https://inform.tmforum.org/open-apis/2017/08/apis-need-good-parents-catalog-success/?mkt_tok=eyJpIjoiTm1aa1pUVXhOR001TkRFMSIsInQiOiJXbEpaajNHRmR1Rm9meTZzQlMzMnJRODJDNlllUjdsdFk2RUxNMDVRS25HMEdlOTZzK3NDNkx5YkZXSjlyQW42eDkrQW5lT0pkRVFpdm5lNXJIdW9STGpaYWV5aHZiald0b1JBenhlSTFRV2FUMVhFNXBLUlRkZ05MV2ZZK1JSViJ9