# CAAP format

_Peios / Using Peios / Wire Formats Reference_

> Where the central access policy wire format is specified.

The central access policy wire format — the structure `kacs_set_caap` accepts — is specified in the **Peios Kernel TRM §3.A**, the KACS ABI reference.

The conceptual model, and what the structures mean, is [Policies and rules](/peios/security-fundamentals/central-access-policies/policies-and-rules.md).

## Shape, in summary

Fields are length-prefixed: each part begins with a 32-bit byte count followed by that many bytes, and an absent field has length zero. That convention runs through the policy blob, its rules, and each rule's applies-to expression and SACL.

The limits that bound it — 256 KB per wire spec, 256 rules per policy, 64 KB per applies-to expression — are in [Other constants](/peios/using-peios/constants-and-catalogs/other-constants.md).

The applies-to expression and the rule SACLs use the same conditional-ACE bytecode as a conditional ACE, specified in PCDS §5.11.

Related content:

- [Wire formats reference](/peios/using-peios/wire-formats-reference/overview.md)
- [Policies and rules](/peios/security-fundamentals/central-access-policies/policies-and-rules.md)