There can be different situations:
1. The outside terrace belongs to the owner of the retail unit. As has been advised in earlier posts, this would be clearly written in the Escritura (Notaria Deed). If it is not clearly written in the Escritura that it belongs to the owner of the unit, then it doesn't. The Escritura could then state who the owner is (for example, the community, the holder of a concession, the Municipality, etc.) but not necessarily.
2. The outside terrace belongs to the community but the retail unit has exclusive use. The consideration as common area would be stated in the Escritura de División Horizontal and registered in the Property Registry. The exclusive use would have to be stated in the retail unit's Escritura. If it is not stated in the Escritura, then the use is not exclusive.
3. The outside terrace belongs to the Municipality or is public domain. As Johnx said, the unit owner can obtain a license from the Municipality to use the space. If the space is public domain, then the license cannot prevent the public from accessing and walking through that public domain space.