One currency, three demands
Amber pays for melee weapon upgrades, for projectile upgrades, and for the partial respec. That shared pool — not skill points — is the actual limit on how much you can experiment, and it is why respeccing casually is more expensive than it looks.
Where upgrading happens
At a campfire, or at the workbench in the cleanse walker. Elemental bolts and arrows come from crafting manuals found in the field or bought from merchants, and enhancing a quiver raises how many you carry rather than their damage.
The campfire menu has four options, not one
Read off the client on 6 August: the Enhance Equipment menu lists Sword Enhancement, Projectile Enhancement and Ranged Weapon Enhancement as three separate tracks, with Spirit Stone Duplication greyed out beneath them behind a lock. Two things follow. Upgrading a sword and upgrading your arrows are different menu entries drawing on the same amber, so "I upgraded my weapon" does not mean the projectiles came with it. And the Projectile line's own help text — "Use Amber to enhance arrows and bolts" — settles something this site had guessed wrong: ammunition is amber-enhanced like everything else, rather than being upgraded only through the crafting manuals.
Not yet written up
The weapon lock system, the blue/yellow/green generator devices and the staged enhancement tiers have been seen but not played through. Spirit stone duplication is now at least located — it is the fourth option on the campfire menu above, gated behind a requirement the lock badge shows as "2" without saying two of what. None of these gets a page until someone has used it.