Starwood Preferred Guest - What points can be transferred to miles?

This is from the airline transfer page:

1Award can only be redeemed with Starpoints earned at Starwood Preferred Guest participating hotels or Preferred Guest earning partners.

What does this mean? What points CANNOT be transferred?

It means that you are not supposed to transfer for example American Express Membership Rewards points to SPG and then convert those into airline miles. In my experience, however, no-one cares and it can be done.

It means that you are not supposed to transfer for example American Express Membership Rewards points to SPG and then convert those into airline miles. In my experience, however, no-one cares and it can be done.
thanks!