What does a tilde (~) mean as annotation for a data pin? When I examine the datasheet for the Arduino Micro (couldn't find the Pro Micro on their site, btw) then I see some data pins annotated with a ~ character, and some are not.

For instance, D9 has a ~ annotation, D8 does not.

What does this annotation mean?
It means the pin can be a PWM output for use with analogWrite().