What is a "semi-selection?"

[Eel_Dodg]

In the manual it says this:

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Select Opaque

Select all opaque (non-transparent) pixels in the current active layer. If there’s already a selection, this will add the new selection to the old one, allowing you to select the opaque pixels of multiple layers into one selection. Semi-transparent (or semi-opaque) pixels will be semi-selected.

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All very clear until the last sentence. How can pixels be “semi-selected?”

Thanks much

[raghukamath]

I think it means the selection will acount for the opacity so when you fill/copy this portion with the selection, the result won’t be hundred percent opaque.

[AhabGreybeard]

Yes, a selection is actually a greyscale mask and painting on a selection (or copying content in a selection) gives paint opacity modified (multiplied) by the local values of the selection mask.
The ‘marching ants’ line that surrounds a selection indicate its maximum extent.
To see the mask values inside the marching ants, you need to toggle Select → Show Global Selection Mask and look at the selection mask in the layers docker.