Connector are used to connect between tubes of different sizes.
When using silicone tubes, it's possible to use a connector that is 1-2mm bigger than the tube's inner diameter, because of their flexibility.
It's also possible to slightly heat up the edge of the tube in order for it to expand it until it fits the connector.
For our uses, most of the work is done using 4mm splitters that connect to 3mm (Inner)- 5mm (Outer) tubings.