There is different types of Baseline in Clearcase UCM:
All UCM baselines are one of the following types: initial, imported, full, incremental, unlabeled or composite, which are detailed below.
An initial baseline is created automatically when a component is created and represents the component in its "empty" state. This baseline is used as a foundation by the project that shall first populate the component with files.
An imported baseline is an alternate way of populating a component. A label that was created in a non-UCM view is associated with a baseline, and the first UCM project can use that labeled set of versions as its foundation.
A full baseline is a baseline that you create by recording the current versions of all elements below the root directory of the component. A full baseline has a corresponding ClearCase label that is applied to one version of every element in the component. This type of baseline takes the longest to create but is the most functionally complete.
An incremental baseline also represents one version of every element in the component. However, its corresponding ClearCase label is applied to only those versions that have changed since the last full baseline. Incremental baselines are faster to create (unless every element in the VOB was modified ) and can be used for all UCM operations
An unlabeled baseline is generally created by delivering from a stream. It is used to determine which versions must be merged to the target stream. An unlabeled baseline cannot be used as a foundation baseline for a stream.
A composite baseline is an aggregation of several other baselines, of other related components. It should be created in a special "rootless" component that is not associated directly with any VOB elements. By using a composite baseline as a foundation, the stream actually has a foundation on all the member baselines of the composite.