When the incoming positive (+) power supply terminal is directly connected to the positive output terminal of a dimmer, this is a common-positive dimmer.  The actual dimming happens on the negative (-) terminal, so the incoming negative power supply goes to the dimmer circuitry, and then the dimmer drives the negative output terminal of the dimmer.  All RC4 Wireless dimmers are common positive, which also means that all the positive (+) terminals on the device are connected together inside.  This is ideal for most LED tape, which is also common-positive: connect the common wire to any Dim+ terminal, and connect each separate color wire to a Dim- terminal.