Click the time-frequency with a current selection.
In the File menu, click:
Load selection to remove the current time-frequency selection (if existing) and then
load the selection from the *.sel file.
The current selection is removed from the time-frequency representation and the selection from the *.sel file is
loaded.
Load selection (join to current selection) to keep the current selection in the
time-frequency representation and then load and append the selection from the *.sel file.
The selection from the selection is loaded and added to the
current selection in the time-frequency representation.
The selection is loaded.
Note: Now, you can isolate and modify the selection.