Every Nord child is told which sign they were born under, and in Skyrim the Standing Stones still stand where they were raised. On Aurbis your Birthsign is a place you have to walk to rather than a menu you fill in at creation, and what it grants you is what your character will be good at for the rest of their life. Choose it once, and choose it knowing there is no second choosing.
How it works
The lesson comes first. /learn is the telling of it, and /stones opens the primer on all thirteen stones, what each one favours and how it plays.
Choosing a sign marks it. It does not bind it. It puts that stone on your compass and sends you walking, and you may change your mind as often as you like on the way. Nothing is fixed until you stand at a stone and touch it, and if you reach a different stone and take that one instead, that is the one you get.
What a stone gives you is skill. A major skill is raised by twenty, which clears that skill's gate outright, and a minor by ten. That decides which schools are open to you and which will stay closed, so read the primer before you walk.
The Divines are separate from the Stones. A blessing taken at a shrine is temporary, /blessing shows what you carry and how long it has left, and while it holds it changes what happens when you die (see Death Has Weight).
The guided quest that walks a new character through all of this is built and waiting to be switched on. The commands work now.
Commands (/help birthsign)
/learn hears the Aetherius Birthsign lesson.
/stones opens the Codex of Stones and the primer on the Standing Stones.
/birthsign shows your affirmed Birthsign, its lore and the skills it granted.
/birthsign choose <sign> marks a sign to seek. Nothing is bound until you reach the stone.
/birthsign cancel cancels a Birthsign selected during this login.
/birthsign cooldown shows daily Stone power readiness, or the cooldown remaining.
/birthsign codex opens your affirmed Birthsign codex.
Blessings (/help blessing)
/blessing shows your current Divine blessing and the time remaining on it.