I know the traditional method of selecting groups of files
Click First File in Set, hold down shift and click last file in the set
Click individual files whilst holding down the Control Key
In the file window is there a way to get a column with a "Tick Box" in for selecting individual files, and not having to hold the Control key all the time. Have looked under General settings, folder settings, and column selection.
You can then go backwards and forwards, view individual files etc without those you have selected, becoming unselected.
You can use the checkbox mode for that. You can find it either in your "menu" tool bar, which you might have to turn on, or you can set up
a button using CHECKBOXMODE=Toggle.
It is similar to, but not quite the same as (and pre-dates) the checkboxes in Explorer.
Also note that you can toggle-select files using the middle mouse button (unless it is assigned a special meaning by your mouse drivers/control panel) which is like clicking with ctrl and the left button, but without having to hold ctrl.
Opus also offers Power Mode which allows you to re-configure the mouse buttons to, for example, do toggle-selection when doing a normal left click.
I'm sure you know that you can select multiple files by drawing a rectangle by holding left mouse button.
In DOpus it's also possible to draw rectangle with middle mouse button, which inverts selection of files within rectangle.