A requirement version is a specific instance of a requirement that was saved at a particular point in time. Any number of requirement versions may be aggregated into baselines.
A collection is a gathering of any number requirements that are all related in some way. A collection may also be aggregated into a document for publishing. A collection may also include requirements that are in a baseline.
A requirement baseline is a frozen collection of versions of requirements at a given point in time. A baseline is related to a collection, and may also be related to any number of requirements approval requests.
A specific type of change request that asks for new requirements or modifications to existing requirements. A requirements change request can be related to any number of requirements.
A request sent to project stakeholder to approve a requirement.
A snapshot is a backup of a specific version of a requirements document.