hwarwick

To replicate:
1. open an image in Affinity Photo.
2. click Elliptical Marquee Tool (or another other Marquee Tool, but Elliptical is how I found this)
3. Select a part of the image with Elliptical Marquee Tool.
4. Add to that selection using the Elliptical Marquee Tool.

Result:
Nope.

Expected:
1. A key command executed in conjunction with Mouse Down that will select part of the image using Marquee Tool without "letting go" of what is already replace.
2. For this key command to be noted through the application of a plus sign (+) next to the Marquee Tool Crosshair.
Note:
Affinity Photo will DESELECT part of a Marquee Tool area using the alt key, which is noted by the appearance of a minus sign (-) next to the Marquee Tool crosshair.
Note: 
This is not unusual for image editing software - Photoshop has this, as does Paint.net and Pixelmator. That one can Deselect part of a Marquee Tool selection shows that there is the ability to edit a Marquee Tool selection negatively. This should be able to be done positively. That it doesn't is kind of embarrassing.

Note:
The following keyboard combos were tested (Results):

Shift (nothing),
CTRL (nothing),
Windows Key, Alt (invokes negative selection),
Shift Alt (also results in negative selection),
Shift CTRL (nothing), Shift Windows (nothing),
CTRL Alt (also results in negative selection),
CTRL Windows (nothing),
Windows Alt  (also results in negative selection).

System:
HP laptop, i7 / 8GB / 1TB, Win 7