Must be a floating point value, such as "1.2".

This may also be a reference to a resource (in the form "@[package:]type:name") or theme attribute (in the form "?[package:][type:]name") containing a value of this type.

[Android Documentation]

Namespace: Android
Assembly: Mono.Android (in Mono.Android.dll)
Assembly Versions: 0.0.0.0
Since: Added in API level 1