Percent Free. The PCTFREE setting controls the percentage of space left unused in a block when rows are first inserted. This extra space is then available for any row expansion required by subsequent updates. A block will be removed from the free list when its free space falls below PCTFREE. It can therefore be regarded as a high-water mark for use during inserts.

Block storage parameter used to specify how much space should be left in a database block for future updates. For example, for PctFree=30, Oracle will keep on adding new rows to a block until it is 70% full. This leaves 30% for future updates ( row expansion).