Change node can set them. other nodes can use them.
Not certain what you mean.
If you define a flow context variable, it is available to every node on that page/tab
If you define a global context variable, it is available to every node on every page/tab
NOTE: Be careful with context. It is tempting to use it everywhere but in truth, it can bite when you have quick successive messages (remember, node-red is async). Where possible, try to put the data to process into the msg, pass it along to the next node for further processing etc. This is akin to PURE programming. A useful piece to export is the link-call node. By its very nature, it helps you minimise code duplication and promotes PURE programming.
