Prb is a derived class (or module for lua) of Probing.
when requiring a module the module name is replaced by what we tell it, in this case its prb, so although you will see Prb.SingleSurfZ its actually running Probing.SingleSurfZ.
seems weird but look at it this way, lets take a module that creates a button called btnCreate and within that module there is a function btnCreate.createButton(coords etc)
we need 3 buttons so we can do this
local btn1 = require btnCreate
local btn2 = require btnCreate
local btn3 = require btnCreate
btn1.createButton(coords etc)
btn2.createButton(coords etc)
btn3.createButton(coords etc)
so btn1 -2 and 3 becomes derived classes/modules of btnCreate.
DazTheGas