SKATBinary example - Z.a. genotype matrix

[Louisa Stern]

Dear SKAT research team, 

I am new to SKAT and I would like to use SKAT_Binary.  I  am struggling to understand what the argument Z.a. in the SKAT_example data is. The argument Z already compromises a numeric genotype matrix. What is this additional "list object of genotype matrices" for?

I would be grateful for your help! Thank you in advance,

Louisa