A subscript is used to refer to some particular entry in a multiple’s dimension while slicing. For example, in the slice:
foo[1,2,3]
The subscript 1 refers to the entry in the first dimension of
the multiple with index 1. This doesn’t necessarily means the
first element: it depends on the bounds of the dimension. Likewise,
the subscripts 2 and 3 refer to the values with indexes
2 and 3 in the second and third dimensions of the
multiple. The action of applying a subscript is known as
subscripting.
When subscripts are provided for all the dimensions of a multiple the result of the slice is an element from the multiple.
Simplified [RR 5.3.2.e]:
e) subscript : meek integral unit.