Sorry if this question is obvious, but for me is still unclear...

My freebsd-version is: 10.2-RELEASE-p25.
I'm considering to upgrade to 10.3 through "freebsd-update upgrade" command.
My doubt/question: what does that command exactly upgrade, only related to GhostBSD? Does it upgrade my GhostBSD (not FreeBSD, I know it does upgrade)? (I mean all customisation, settings, closely related to GhostBSD).
...my guess is that so far the only way to really upgrade GhostBSD (not FreeBSD) is through DVD... is it correct?
An other question on "freebsd-update upgrade":
Since mother distribution (FreeBSD) is to version 11.X, is it convenient directly upgrade to this major version?