Specifications

The specification is a document that describes the behavior of the software system to be built. It contains the requirements model and the domain model. The requirements model includes functional requirements (capabilities) and non-functional requirements (constraints).

Functional requirements are often described with UML use case and sequence diagrams.

Non-functional requirements include:

Usability

Impedence mismatch with existing workflows
UI conventions
online help
documentation

Reliability

mean-time between failures
fault tolerance/robustness
safety

Performance

Response time
Throughput
Availability
Accuracy

Supportability

How easy will it be to add new features? fix bugs?

There are several standard specification documents including the Requirements Analysis Document.