I/O (input/output) nodes are in a sense the "anchor" of a zone.. as well as providing a means to enter or leave the zone in question.

a node is essentially the core of a base once a zone has been stabilized, and (supposedly) well defended if the base builders know what they are doing

There are two major forms of nodes... the "major" ones are those that are used to enter or leave the zone if you are not going to use a Hack to do the same... but some hacks for "gate hacking" can sometimes make minor nodes as well...

put simply a minor node is used for things inside the given zone, such as getting into a mission "dungeon" or some place for an instanced episode.

In the case of gate hacking, sometimes going in thru a major node inside a base would be a way of "getting in thru the back door".. but as I stated earlier, the node would (obviously) be heavily defended. Some forms of gate hacking, however, might be a bit smoother... in one case, the hacker can make a momentary minor node to get inside a less secure part of the base, or near the base they want to hit.... the only drawback would be that this node would only be "one-way", and would vanish once all the members of the group were thru.