The Page Inventory Page (PIP) is one of the ten page types defined in Firebird/InterBase®. The PIP is used along with the pointer page for space management.
Every page in the database is represented by one bit in the PIP, this bit indicating whether the page is currently in use. PIPs occur at fixed intervals in the database, the interval being determined by the database page size. PIPs are never released.
For those interested in more detailed information, Ann Harrison's article, Space Management in InterBase®, provides an in-depth insight into page types and their roles.