Please teach me why wallfilter is IIR.

My name is Masatoshi nakashima,

I am learning ultrasound signal processing.

Please teach me about  wallfilter algorithm.  

In Application Report SPRAB11–November 2008
"Efficient Implementation of Ultrasound Color Doppler
Algorithms on Texas Instruments’ C64x™ Platforms",
there is a explanation of wallfilter.
"This module does an IIR filtering operation.
The basic operation done for filtering for each scan-line set is a multiplication of complex input
data matrix, X, with real coefficient matrix, W, given by,
Y DxN =X DxN  *    W NxN      "

I can not understand why this equation expresses IIR.  I think IIR should have feedback input.
(1) Please teach me, what is feedback element in this matrix conversion equation.
(2)Please teach me, how to design W matrix. 
Please  pardon me for my  poor knowledge.
regards.