This is from Bhikkhu Bodhi’s In the Buddha’s Words introduction:
The Abhidhamma Piṭaka is obviously the product of a later phase in the evolution of Buddhist thought than the other two Piṭakas. The Pāli version represents the Theravāda school’s attempt to systematize the older teachings. Other early schools apparently had their own Abhidhamma systems. The Sarvāstivāda system is the only one whose canonical texts have survived intact in their entirety. Its canonical collection, like the Pāli version, also consists of seven texts. These were originally composed in Sanskrit but are preserved in full only in Chinese translation. The system they define differs significantly from that of its Theravāda counterpart in both formulation and philosophy.
Source: https://www.wisdompubs.org/book/buddha’s-words/introduction