You can access your V-Matrix Skills via the V-Matrix button in your 5th job tab
Each node slot in your node window can be enhanced upto 5 times. This increases the cap of your 5th job skills past their original level cap. You can drag your skills to each of these slots to use on your 5th job skill tab. Each slot can be purchased for mesos or unlocked by its level requirement.
To upgrade or unlock more nodes talk to the npcs in Chuchu or Nameless town
How to Create NodeStones
Here you can craft your nodes via crafting button
Crafting nodes requires Node Shards which can be earned from disassembling unused nodestones
Press disassemble nodes and select each node you would like to disassemble for Node Shards
You can also craft a nodestone which are tradable nodestones that can be sold in auctionhouse or given to another character. These nodestones provide a random skill when used.
How to upgrade/enhance nodes to higher Levels/Ranks
First talk to the NPCS in Nameless or Chuchu to access your nodes once again then select a node you would like to enhance.
To enhance a node right click on it and press Enhance and choose the same nodes as the one you are enhancing
What is a TrioNode?
A trionode is a combination of 3 of your 4th job skills in one node. These are really important as they boost your 4th job skills by a large amount when maxed. TrioNodes can cap upto level 60
Recommended TrioNodes Guide (Reddit)
After maxing your important TrioNodes next up are your main 5th job skills which can be capped to level 30
For anymore information on Vmatrix and nodestones heres another guide