Say I am applying a Blur TOP for example, and I want it to act only on the top part of a TOP or of the scene, so I want to mask the effect, with feathering so that it softly transitions from affecting the top part and less so towards the bottom and nothing at the bottom,
whats the best way to mask in such a way?
thank you 
so far I am using a composite TOP in multiply mode to multiply whatever TOP with a black and white texture mask created with a circle or a rectangle TOP or similar that I position wherever I like; is that the best way
The Matte TOP is intended to help with this sort of thing by giving you 3 inputs, input1, input2 and the mask input that crosses between the first two based on a specified channel of the third input. But this doesn’t actually affect and effect’s intensity, it just does local crossfading/matting. Local per pixel effect intensity control would need to be done by customizing the effect in GLSL or in the case of a blur, you could use the luma blur TOP which blurs based on a second input’s luminance or other channels.
Specifically about blur, you could look into the Luma blur TOP.
Connect your image to one input and a black and white ramp to the other input.
The operator snippet might have some examples
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@archo-p @FaustoB thank you both, yes I knew already about the luma blur, and the Matte TOP I had heard about thank you, but really what would be most useful is to be able to change the degree of impact of a masking effect; surely with GLSL but I would think there would be some other way without having to code it in glsl
Unfortunately ( or fortunately depending how you look at it!) you will need to get into GLSL to achieve some of this stuff beyond the Luma Blur and Matte TOPs. It’s certainly an interesting RFE for channel control of stock TOP filter parameters to ask of Derivative, but the fact that most of the common effects are possible to find code for will give you the most ability to control this stuff ASAP, welcome to GLSL, think of it as inevitable! The wiki has some pretty great advice on writing GLSL in TD as well as some of the op snippets and TONs of posts on this forum, it’s not as daunting as you might think
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@archo-p I know, I know, Ive done plenty of GLSL already, got you, just thought that such a basic and useful operation would be part of some kind of prepackaged node already but ok I see that this is not the case, thank you