I read in the documentation box of some effect (typically those whose name ends with "(mask)") that: "... The typical use is to add effects in the following sequence: this effect, zero or more effects, mask_apply. (mask_start)".

To make a test, I tried to use a simple "Alpha shapes (mask)" followed by the "Mask apply" effect.
The Alpha shapes effect works just fine (I see both the top and lower layers images correctly), but when I add the "Mask apply" effect I can only see the top layer image.
If I disable it, then I can see things correctly again.

...I am not getting what is the purpose of such an effect.
Any hint/examples?

the goal of 'mask apply' is to apply effects to part of a clip only. Say you have a red color clip, you add first rotoscoping, and make a shape, then greyscale. Now you have a grey shape with black around. If you add mask apply you now have a grey shape with red around. And of course it works with any clip, effect and shape

Arkengheist wrote:the goal of 'mask apply' is to apply effects to part of a clip only. Say you have a red color clip, you add first rotoscoping, and make a shape, then greyscale. Now you have a grey shape with black around. If you add mask apply you now have a grey shape with red around. And of course it works with any clip, effect and shape

Ah ok! I got it now, it was not obvious from its description to be honest

Just to double check if I got it right: I can get the same effect by duplicating and overlapping my clip on itself (say the red color clip) and then by "rotoscoping" a shape and by adding a greyscale effect on the top layer. In this way we should have a grey shape with red all around, right?
But with the Mask apply effect we achieve the same without the need of duplicating the clip on an above layer but doing everything in the same layer.
Correct?

yep, saves you a track