From Synfig Studio :: Documentation
If the "Lock Selection" parameter is turned 'On, then:
- If you click on a grouped layer in the WorkArea window, the Group Layer will be selected, rather than the layer you clicked on. For example, you draw an outline, and then group it. By default the "Lock Selection" is Off, and clicking on your outline in the WorkArea will select the outline layer. With "Lock Selection" On, the group layer will be selected instead.
- Right-clicking on a group layer offers a context menu entry to "Select All Child Layers", which recursively selects all the layers under the current layer. If the "Lock Selection" is engaged for any group layer in the hierarchy, it stops the layers inside that group layer from being selected by this operation.
- Hitting to select all the Handles in the currently selected layers will select the Group Transformation Widget's handles of selection-locked group layers, whereas it won't when their "Lock Selection" are Off.
The effect of these three behaviors is for a group layer to act as if it was a primitive layer, hiding the details of its contents from selection or manipulation.