How does it work? You buy ticket and can seat anywhere on the grass or your ticket specify the exact location?
In ODI's, you get a ticket allocated for a particular seat and particular stand but if you feel like it, you can wander out to the grassy knolls as u are allowed (as marked seating costs are higher), but if you buy the grassy stands, costs are significantly lower, but you cannot got to allocated seating. (well you can, once the match starts and after lunch/first innings once every one gets a bit 'high' and if you can manage to charm the stand attendant

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In T20 -chances are very less -as more hectic and its not as relaxed.
In tests - test matches are more relaxed than odi, you could pretty much go anywhere and everywhere except the VIP stands and press boxes - and we used to love sitting in the grassy knolls - as the more 'colorful' guys who had 'opinions' on the game would be found here...
to answer ur question specifically, on grassy knolls you can pretty much sit anywhere....and tbh its one of the best experiences....you just bring a beach towel or mat, stretch your legs and if you have good company and the day is good, its awesome experience...
The awesome kicker is in NZ in tests once lunch/tea is called, once teams walk off, you can actually go on the playing ground, (excpet the pitch off cours, is cordoned off)....its a massive buzz walking on the same ground that these guys play....