What does this notation mean?

Dear Comsol users
In the document

http://www.pksez1.com/Modeling_PIN_Photodiodes.pdf

which describes the simulation of a PIN diode, the author writes in Table 2 for the doping concentration N the expression:

N = NDn + NDnmax*exp( -((y+y1)/ch)^2 ) - NApmax*exp( -( y/ch)^2 )*( (abs(x) < ac/2) + (abs(x) >= ac/2)*exp( -((abs(x)-ac/2)/ch)^2) )

I would like to ask how to read the "conditional part" containing the comparison operations '<' and '>='. These, logically, evaluate to 'true' or 'false', in my world.
What kind of effect do they have here?

Thanks for any hints and advice.

Martin