What is this bolt for?

[Baxter]

Can anyone tell me what this bolt is for? 2019 Yamaha T9.9.

(I originally thought this was for the trim fluid fill, turns out I was wrong and it is the slow screw below - but I had tried to turn this bolt to check and it is totally seized. Trying to decide if I should care or not...)

Can't find any repair manuals online that will give me my answer. Might bite the bullet and buy one, but for now, you guys will do

 

[ship happens]

It's a plug

Something to do with the assembly of the ram. Probly something in behind it. As part of the assembly process. Or just a plug for the expansion tank. The factory calls it a tapered plug. Aka NPT plug