Before we get down to Filters and Rules one important term must be explained: a connection. It is a single network (internet, local network etc.) connection which allows to transfer data between two connected machines. It can use several communication protocols like IP, TCP, UDP and others, but what is more important from NetLimiter's point of view is a direction of connection. There are two directions:
INCOMING: It is created when a computer (or program on that machine) accepts incoming connection request from other machine. In order to be able to accept any incoming connections, the computer must listen for them. In Activity view, they are represented by red arrow.
OUTGOING: When computer initiates a connection then it sees it as outgoing. (The opposite machine sees it as incoming) In Activity view, they are represented by green arrow.
As you can see, the direction does not tell you which way the data flow, but how it was created. Regardless of the creation data can flow both ways (sent or received) for most network protocols.