Final answer:
An agency has the greatest discretion in implementing policy typically after enactment and during the detailed rulemaking phase. Agencies use their expertise to fill in legislative details, especially in bottom-up implementation models that allow for more localized flexibility. Public input is also a factor that can influence how policies are implemented by agencies.
Step-by-step explanation:
An agency has the most discretion in implementing policy usually after the enactment stage, where legislation has been passed but specifics of policy implementation are not fully detailed. During this phase, agencies often partake in federal negotiated rulemaking processes to clarify grey areas and detail how new policies will be implemented. This is because Congress generally lacks the capacity to legislate with the granularity required, hence experts within agencies take on the task of devising the procedural nuances.
The level of discretion also depends on the implementation model; with a bottom-up implementation approach, local agencies may have more flexibility in tailoring the policy to meet specific local needs, whereas a top-down approach dictates stricter adherence to federal guidelines.
Furthermore, the opportunity for public input is frequently mandated in regulatory changes by the federal government, which allows for the consideration of external viewpoints and public opinions in the policy implementation process.