In SAIL Recipes ,following code snippet is there for desigining inline editable

In SAIL Recipes ,following code snippet is there for desigining inline editable grid. I have a question related to the partial function used here. For the supporting rule "ucItemRowEach" ,there is a parameter called "index". What value goes into this "index" parameter during run time.How does this partial function resolve during run time ; as per documentation we need to provide parameters for partial functions in a paranthesis following the function.

rows: a!applyComponents(
function: rule!ucItemRowEach(
items: local!items,
index: _,
itemsToken: local!itemsToken
),
array: if(or(isnull(local!items), count(local!items) < 1), {}, 1+enumerate(count(local!items))),
arrayVariable: local!itemsToken
)

OriginalPostID-155367

OriginalPostID-155367