There are two intrinsic functions to calculate for an XAFS path. The debye(temp, theta) function generates for a path given a temperature temp and Debye Temperature theta using the correlated Debye Model implemented by Rehr, et al. in FEFF. Similarly, eins(temp,theta) calculates for a path given a temperature temp and Einstein Temperature theta using the Einstein Model.
Of course, these two functions need more information than just external temperature and characteristic temperature. All of the other information depends on which XAFS path is being considered, and is found using the concept of the Current Path, further discussed in Section 7.1. Without repeating too much of that discussion here, the basic point is that IFEFFIT keeps track of which path it is considering with the variable path_index. Within the functions ff2chi and feffit(), this variable is looped through the paths you specified, and the path-dependent values like reff are automatically updated. Similarly, if a Path Parameter depends on debye() or eins() (even indirectly), it will automatically be updated. From outside ff2chi() and feffit(), you can set path_index yourself, and then both debye() and eins() will use be evaluated for the specified path.