Question:
Why is there a new 'Lab' option included in the rgb workflow for CWS 5.0 yet it can't be selected?

Answer:
Unlike CMYK or RGB color spaces, the Lab color space is a device independent space, thus there is a clear understanding of what Lab is. When a job enters a Fiery in the Lab space we already know what Lab represents. This is in comparison to a job in RGB space where the RGB could be Adobe RGB, sRGB or scanner RGB; we need the RGB source profile to define this device dependent color space.

Lab is processed like RGB except for the source RGB profile.