beq parameter in TOPAS Academic

[sisir ray]

 Dear All,
I am a TOPAS academic user and I was wondering what is the significance of
"beq" parameter in the refinement process.In other words how should I change
it to get started in refining a structure.As the examples suggest I can get
started with 1.But should I change it 0.5 ,If I have two element at one site
i.e. for systematic substitutions.Thank You for your help.
Best
Shishir Ray