Dumb question .. so you aren't asking for a job at the agency? a Rather asking to be a subcontractor when they get a gig? A bit confused here.
It is an attempt to stop employers dodging benefits entitlements by hiring their employees as contractors.
0: https://en.wikipedia.org/wiki/Permatemp#Vizcaino_v._Microsof...
In essence it means that you can't take paid leave or sick days, otherwise it counts as "disguised employment". Another interesting requirement is that you must have at lest one other person in your company who can reasonably stand in for you and perform your work, if needs be.
If caught on the wrong side of the law then you must return the tax you've saved along with paying a fine. Not nice.
Generally it's just a matter of how the contract is worded. You need to reserve the sort of rights a business would reserve.