This menus groups operations which filter images, or which are filters in the photoshop sense.
Two menu items are slightly more complicated. Unsharp Mask transforms to CIE LAB colour space, then sharpens just the L band with a cored unsharp filter. The Tasks / Print menu has a version of this filter tuned for typical inkjet printers.
Custom Blur builds and applies a square or gaussian convolution kernel for you based on a radius setting.
The Threshold item does a simple level threshold. Use the Math / Relational menu to construct more complex image binarisations. Use Math / Boolean to combine morphologies.
The first half of the menu lists simple erode and dilate operations, 4- and 8-way connected. The second half contains several useful compound filters.
See also Histogram / Find Profile for something that can search an image for object edges. And MathStatistics / Edges can count the number of edges across and down an image.
You can make other mask shapes yourself using the Image / Make Patterns menus, then apply them using Math / Fourier. You can also use the image paintbox to directly paint out peaks in a fourier-space image before transforming back to real space.
One useful version is to use a text image (see Image / Make Patterns / Text) to blend between two colours (see Colour / New).
Along Line does a left/right or top/bottom fade between two images.