ZSE Center Controller reported they issued a route revision via CPDLC and the pilot deviated from the route.

Narrative

Aircraft X departed PDX and was handed off to Sector 06 on a 200 heading. W570/EEL complex was in use and the heading was enough to clear it. I wanted to give a fix to route around it so I could hand off to high altitude without releasing control. The next fix on Aircraft X's flight plan was ONP but going direct would still conflict with airspace so I sent a new route of COGOK..ONP as filed via CPDLC to go around the airspace.Aircraft X turned direct ONP; then about 15 seconds later acknowledged the CPDLC route. They then called and asked about the free text in the message that had COGOK in it and stated they didn't know what that meant. I turned them back to a heading of 190 to remain clear of airspace and explained that the route should be COGOK and then ONP; not direct ONP. The message out on my CPDLC menu stated 'cleared to ONP via COGOK; due to airspace restriction.'Pilots still seem to be struggling with these reroutes and just seeing the fix they're cleared too and disregarding how they are cleared to it. Airspace was never violated and the situation was okay; but I want CPDLC to be a tool that I can use to help when I'm busy. Instead; it seems that for the more complex functions it turns more into something that I need to constantly watch to make sure the pilots are doing it correctly.I would recommend either better training for pilots on how to read and load these routes or a change to how the routes are displayed. It seems like it would make more sense to have the route come across in plain language like a verbal clearance would be given; such as 'cleared direct COGOK; direct ONP; rest of route unchanged.'However; it is resolved there still seems to be confusion that needs to be straightened out.