Pctused and Pctfree are block storage parameters.
Pctused is used to find out how much percentage of the block will be used to store rows.Pctfree is used to find out how much percentage of the block will be used to store rows resulting from further updates to the rows in the same datablock.

Eg. If u keep pctused to 40% and pctfree 20.so u can insert rows till 40 %.if the limit exceeds 40%,still also u can insert rows in the datablock till the limit reaches 80% (100%-20%) as u have kept pctfree to 20%.Now if one goes on deleting the rows,the block is not said to be free unless and until pctused falls below 40%.As soon as pctused falls below 40% from deleting the rows, that block can be used to insert the rows.In this way the cycle continous.So it is recommended that u never sum up pctused+pctfree=100.Always have some gap between them this helps in reducing ur Oracle server for allocation and disallocation of freelists.

If any further doubts write to [email protected]