Choosing a Basis Time

When you add a stream to a depot, you have to specify its basis time. Among other things, the basis time affects whether or not a stream continues to inherit versions from its parent stream. There are three types of basis times you can assign to a new stream:

None
No basis time. The new stream inherits the versions currently in its parent stream and continues to inherit subsequent versions that are promoted to its parent. This choice is disabled if you specified Snapshot for the stream type.
Now
The basis time is set at the time the stream is created. The new stream inherits the versions currently in its parent stream but does not inherit any subsequent versions promoted to its parent. Streams with a Now basis are similar to Snapshot streams in this regard, but you cannot promote into a Snapshot. You can promote into a stream with a Now basis.
As transaction #
The basis time is set to the time of the transaction number you enter. A transaction is the record of a particular change to the repository. The new stream inherits the versions that were in its parent stream at the time the specified transaction was created.