I am not so sure it is possible for anyone to truly explain connectors, but here is a very basic example that i've shrowded a ton of caveats from for the sake simplicity. I add that Fabrications sandbox method of connectivity (while difficult to understand) has empowered me to achieve some pretty epic results I would not expect from any other piping/duct software available. Without building an assembly I've been able to design a 5 (unique) object connection capable of a takeoff as cut-in/stretch populate perfectly.

A Duct Object

Connector 1 Name: Duct Flange

Connector 1 Type: Male

Connector 1 Connectivity: FLG

Connector 2 Name: Duct Flange

Connector 2 Type: Male

Connector 2 Connectivity: FLG

If you haven't disabled error messages (edit configuration dialog) trying to put just 2 of these together should give you a connector mismatch. Basically your trying to put a male on a male and in order to "complete" a connectivity chain there would have to be a female connector involved. In comes the gasket.

A Gasket Object

Connector 1 Name: Duct Gasket

Connector 1 Type: Female

Connector 1 Connectivity: FLG (notice it is identical to the male version....... its mate)

Connector 2 Name: Duct Gasket

Connector 2 Type: Female

Connector 2 Connectivity: FLG (notice it is identical to the male version....... its mate)

If just these 2 objects exist in the service you should be able to keep clicking the duct and it will just keep pairing more of them together with a gasket between them. It is a complete connectivity chain both in sex and connectivity names; size is a factor too depending on what your doing.

If i remember right setting the connector type to "None" will alter the rules of engagement and potentially allow the direct paring of a duct to duct without a gasket. May be more involved then that, but I can't recall since I avoid that method like the plague.