In UCM clear case, when a developer checkis in code in his development stream, he makes a 'deliver' operation whereby the changes are made available in the integration stream.
In base clear case, after developer checks in in code, how can he make it available in the main branch? I know he can merge it using Merge manager, but is there a default way of doing it? Merge manager is a manual process.
For one, the deliver is also a manual process.
What is "automatic" is the Stream to which the developer merges from the child Stream.
By default it is the parent Stream, which happens to be the "Integration" Stream if the developer Stream is a direct child.
See "Integration stream vs integration view in ClearCase".
Now there is no easy way to establish a clear relationship between a branch (used by a Base ClearCase view) and another one (including the /main one).
That is why UCM exists: to introduce some best practices around SCM processes, like merge workflow (merging from where to where?).
That is why the official way to merge from a "sub-branch" remains findmerge, as described in the documentation.
