I think she means why on earth would you want to be exclusive with iStock?
It's hard to know. For a while, it was 500 sales. Then they said anyone who wanted to could become exclusive (so long as they had no RF files being sold anywhere else and didn't give away files from any site), however there seemed to be a time when portfolios were being reviewed and I even read of a couple of people being refused apparently on the grounds of their portfolio's commerical appeal. Allegedly. Still, the situation can change so quickly, you should check for yourself.
Go to your Account Management page in ISP, click on Agreements, and apparently/allegedly if you can access the page called 'Exclusive Content Agreement', you qualify for exclusivity.
All that said, like Shelma, I don't recommend it. I'm exclusive, but only because I don't fancy any of the alternative. I'm not uploading, though. They change their minds and break their promises with alarming regularity.