As far as i know, Use Case Specification is basically Pre-Condition, Success Scenarion, Alternatives and Post-Conditions. The Specification basically specifies or describes ways to fulfill the user/stakeholders requirements.
2. What is Use Case Realization
As far as the Use Case Realization concerned, It basically inspires to come up with Conceptual Domain Model (i.e., being able to identify concepts in the domain - classes) and design Model. More specifically, the Use Case is realized by developing Domain Model - creaing Class Diagrams and Design Model - with Interaction Diagram and other artifacts. The idea is that all the specification defined in the Use Case are met/acheieved.
I would like other experienced veterns to comment on it for more clarification. thanks.
A use case realization is a RUP artifact. It's basically the collection of artifacts which describe the design which fulfills the use case specification. Frankly I think it's an artificial concept and have never found any use for it in practice. However, if you find value in it then go for it.