"Even a character with a special resistance to magic (for example, an elf’s resistance to sleep effects) can suppress this quality."
I did not know that.
You seem to be right.
Its an obscure rule, i just happen to know it since i just read the PhB cover to cover again 2 days ago.
The problem i see with that is... It's not SR... It's an actual immunity to the effect
Example: An undead cannot give itself a fear effect, even if it is voluntary. It is simply immune to fear. Or heal itself with a cure spell voluntarily, since it is harmed rather than healed by positive energy.
Or an elf accept a sleep spell, since he is immune... Ohh wait.. Thats the example.
How is one immunity different than another? Should they have list all immunities in the game, or should they make an example you can draw from. For me this seems rather simple and easy to understand. If you have a special resistance, such as an immunity to something, you can suppress it.
Healing an undead with positive energy is a bad example, since that is a replacement effect, not an immunity.