Pretty much that stuff above. You do some certain projects (say...unlock Mk XII Borg prosthetics). Once the project is done, the store will unlock, and you can buy (for dil), the Mk XII versions of the Borg arm weapons.
Its a dilithium store that requires a project to open it up. It just has regular Romulan Space and Ground Weapons, Borg Ground Weapons, and anti-borg combat consumables (assimilation hypos, grenades, etc). So basically you spend a certain amount of omega marks and other stuff for the ability to buy stuff with dilithium. You could equip your entire away team with Romulan weaponry or outfit your ship with Romulan (Plasma Disruptors) Weapons.