Z - Category Stocks

Can somebody pls explain to me what is a Z-category stock? or give me a link to read about it?

I use IIFL for trading, whenever I try to place a buy order of a stock from a Z-category they say that, I cannot place it via from the web-browser and the only way to buy them is to place an offline order with them (that as I understand is they will buy for you).

But since they will buy it for me, I am concerned about being hit with the slippage.

Are there any other brokers who let you buy Z category stocks online?

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Thanks..
IIFL Customer Care