The combination of "enter" selectin in the option "update"

D3 documentation says:

The input selection merges with the update selection when you add or paste. This approach reduces code duplication between Update. Instead of using operators for both input and update, the selections are separate, now you can apply them to the selection of updates after entering the nodes. In the rare case that you want to run statements only on update nodes, you can run them in an update before entering new nodes.

I do not understand the meaning of the phrase merge intoin the first sentence. Can someone explain this bit? (Perhaps I lack connectivity with some standard database terminology?)