What does this compliance item mean?
Captions are helpful for those with auditory impairments, and audio (spoken) descriptions are very helpful as video descriptors. However, sign language adds much more depth to the media; it has expression, personality, and dynamics that simply aren't possible with captions or audio descriptions. This level of compliance is considered Level AAA (the highest level in the WCAG 2.0 specs).
When would this be used?
Say you're doing a webcast, and you're providing a signer. They could appear in a small box on the bottom of the screen. Easy breezy!
Event or speech
You're involved in a corporate event, presentation, or speech. An interpreter could be at the corner of the stage. If it's recorded, ensure the camera operator captures the interpreter in the shot.
How can this be implemented?
There are many interpreters that are available for both onsite and remote (e.g. webinar, etc) presentations. Many use technology such as video remote interpreting that facilitates this service. As for costs - it's not cheap. Most VRI services will run between $2-4 a minute, but it's well worth it to make the content accessible for those experiencing impairment.