Ideal two dimensional step function

I am currently implementing a 5x5 kernel suing the following ideal two-dimensional step function:

My first question is: Is this ideal 2D step function based on Heuckel's algorithm?

This function is geometrically represented as follows:

I have generated the 5x5 by iterating the rows and columns up to 5 units. However, I am still trying to understand the geometric representation. Given I want to construct a 5x5 kernel and 'a' here denotes the diameter, what would be the radius? And what is 'h' in 'a+h'. Overall, how can one interpret this geometric representation?