These are the currently 3 tiers of import systems.
Note : All MSSQL importers, or MSSQL within systems that also have MySQL support are all classed as 3rd tier
Tier one systems are fully supported by the support team and development, they are the most active and common imports that customers do.
These importers were either upgraded or have been recently developed as all new importers are tier one.
Systems in this tier are the less common imports and legacy systems. Tier 2 systems can be promoted to tier 1 if there is enough demand, or retired to tier 3 when the source system reaches end of life or demand falls below a level that makes their existence viable.
This is the graveyard of importers, these importers are not supported. There are three reasons imports make it here. Firstly is because their source system has reached it's end of life and the source data model isn't changing any more so there are going to be no future updates. Secondly, that demand for them is so low that it is not viable to offer support for them as it makes no business sense (though the importer is here for people who want to delve into it).
Thirdly, that the source system is such a challenge to import from that the time typically taken for each import far out weights any sensible decision to try to support each individual import that would take hours per client with customisations.