Other is not the right one to use unless it matches the connectors in the families, which it probably won't. Other is generally used for gas, medical gas, compressed air, etc. People seem to be getting too hung up on the basic system groups. Although sanitary may make you think of dirty water drainage, it is just a name used to cover drainage. Therefore it is the most suitable for rainwater drainage. It only does basic discharge unit calcs so no good for rainwater. However when scheduling the drainage systems, you will get all the types in your project in one schedule rather than using two types of schedule. So, with regards to adding to your families - no need to worry about it as you just need to make sure that the basic system group type is applied correctly. I.e HVAC piping use Hydronic supply or return, SVP or RWP use sanitary, Vent pipes use vent, etc. You also need to make sure that you set up the various system types with the correct basic system groups in your project & template.
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