Understanding terms in daily margin statement

I sold a stock named ‘X’ for a market price of ‘Y at date T’.

In my daily margin statement on T+1, there is a section called details of POOL valuation, under which a haircut percentage of ‘Z’ was applied to ‘Y’ and the resulting amount was available in my limits section.

What does this mean in practice ?
Can this amount be used to buy other stocks ? what about the rest (Y-ZY) ?

You can check out this article for detailed explanation of the margin statement: