Does the candidate qualify for a Belgian Work Permit?
The only explicit requirement is that the position is "high level" – commanding a salary in excess of BGF 8200 per month (cGBP 1366) if paid in Belgium, or BGF 140000 (cGBP 2333) if paid elsewhere.
It must be shown that the candidate has the qualifications and experience necessary to fill such a high level position, but there is no need to advertise for the positions.
If the application is for an assignment permit (b above) then the candidate should have at least 6-12 months experience with the service provider, to justify the claim that they have experience with the company's systems.
NB A Belgian work permit is employer specific, and working for another Belgian company would not be legal unless and until a new application had been made by the new employer and approved by the authorities.