Managing Black Belt Candidates

I am looking for information on the selection critieria used by companies to select black belt candidates. 

• are candidates limited to "high potentials"
• is the grade level limited (e.g., lower level grades)
• is there a tenure requirement (e.g., 1 year or more with company before being nominated)

Many companies use black belt assignments to develop their "high potential" employees.  I am interested in how your black belts are being evaluated while in this assignment.

• minimum process improvement project savings,
• number of projects completed,
• time to become a certified black belt,
• project team reviews, etc.

Also, what is the term of the black belt assignment (e.g., 1 year, 2 years) and when does it start (when assigned, when certified, etc.)?

Thanks for your help.