From Teletraan I: The Transformers Wiki

A Powerlinx symbol from Energon. Transformers with same-colored Powerlinx symbols can combine with each other. (Ironically, transformers with a red Powerlinx symbol, as depicted here, are the exception to this rule.)

The term powerlinx has several different meanings, most of which have to do with robots combining with one another. In the Armada franchise, powerlinxing was the act wherein a Mini-Con attached to a Bulk, adding a boost of extra power to the larger robot.

In the Energon franchise, powerlinxing was the process by which two similarly sized Autobots combined (via the Spark of Combination) into a single, more powerful robot.

Cyber Planet Keys do not Powerlinx. Well, except Demolishor's. And deluxe Optimus Prime's.