... generated by predecessor processes
Project Scope Statement
Project Management Plan
... introduced by external units
Enterprise Environmental Factors
Organizational Process Assets
(5.1.2) Process Definition
Quality Planning is the process for "identifying which quality standards are relevant to the project and determining how to satisfy them": Quality planning means planning howto fullfill process and product (deliverable) quality requirements: "Quality is the degree to which a set of inherent characteristics fullfill requirements" (comp. PMBOK3, pp. 179f, 183)
By planning the quality one has to respect some principles:
Customer satisfaction comes first: Quality is defined by the requirements of the customer.
Prevention over inspection: It's better to avoid mistakes than to inspect the result and repair the defects.
Management responsibility: Costs of quality must be approved by the management.
Continuous improvement: Becoming better is an iteratively structured process.