By default, every SNiFF+ tool is reusable. Whenever a tool is needed, SNiFF+ tries to use an open tool of the type requested to avoid cluttering the screen.
You can, however, freeze any SNiFF+ tool by selecting the Frozen check box in the tool's status line. SNiFF+ cannot then reuse the tool to comply with a new request, and has to open a new tool. This can be useful when you want to compare results. To see this: