Designer has a number of alternative fixture selection methods on the layout plan view to allow for easier selection, as well as altering the current selection.
To see the selection method options, right-click in the space on the plan view (not a fixture) and you will see the following context menu:

The four selection methods are as follows:
Normal selection is the default selection method. Clicking a fixture or dragging a lasso will clear the current selection and add the new fixtures to the selection.
Additive selection can be done by holding CTRL/CMD under normal selection but can be toggled to this mode here to allow users to add new fixtures to their selection without having to hold any additional shortcut keys. This is useful for selecting groups of fixtures in various places around a layout without the risk of losing their current selection. For example, selecting every other grouping of fixtures around a Ferris wheel would be much easier to accomplish with this feature.
Subtractive selection is the inverse of the additive selection. Every fixture that is clicked on or included in the lasso will be de-selected from the current selection. This is useful if you accidentally included fixtures you didn’t mean to, want to tweak a selection for a different task or if you want to remove a cluster of fixtures from the currently selected fixtures.
Invert selection will cause any fixtures that are not selected to become selected, or selected fixtures to become deselected. This is useful if, after using the other methods to select half of your fixtures, you want to select the other half instead – you can turn on this mode, then lasso around the entire layout, inverting the selection you had before.
There will be occasions where, with a composite or multi-element fixture, you may want to select elements within that fixture. This is possible using the ALT key. Holding ALT, then starting a lasso, you can then select the elements within the multi-element fixture, as shown below:

This then works with the other selection methods, so with additive selection, holding ALT then clicking/lassoing on elements will add those elements to the selection. Holding CTRL and ALT has a similar effect.