REST API - What are High and Low in effectiveBasePermissions and getUserEffectivePermissions?

When you call the REST api on a web or a list using effectiveBasePermissions or getUserEffectivePermissions, you get a response back that includes two properties, High and Low, that return numbers.

Does anyone know what these are exactly? Are they enum flag combinations of the SP.PermissionKind enumeration? I can't find any documentation on SP.PermissionKind, and the available values for it.

{"d":{"EffectiveBasePermissions":{"__metadata":{"type":"SP.BasePermissions"},"High":"2147483647","Low":"4294967295"}}}

Thanks,

Adam