In general, the Index port of Select in Array can take either a single value or an array of values.
It doesn’t say that on the reference page, but if you dig into the user guide, you’ll find that info. Or, like many people, you’ll find that out by just trying it, or seeing it in someone else’s ICE tree.
The Index port is an example of a port that can change its structure type. The structure of a port is either single or array.
Sometimes, what’s downstream from Select in Array will determine the structure of the Index port. For example, if you plug Select in Array into the Distance Value of Curve Distance to Curve Location, then Index must be a single value (because Distance Value is a single value).