I am mentioning the definitions of both PCTFREE and PCTUSED parameters

A PCTFREE is a partition of the data bolck that is not filled by rows as tehy are inserted into a table, but is reserved for later updates made to the rows in that block.

PCTUSED is the percentage of space in a data block, which ORACLE attempts to fill before it allocates another block.

Hope I am also not sure about these parameters, But for you, I refered for these...