There are several methods you can use to select clips in the Source Area.
To select a single clip:
The border of the clip proxy turns green indicating it is the Source Area focus clip and that it is selected.
To select several clips:
The border of the last selected clip turns green; the borders of other selected clips turn yellow. The clip with the green border is both selected and the focus clip. Clips with the yellow border are only selected.
To select all clips:
All clips in the Source Area are selected.
To unselect all clips:
To select ganged clips: