Hi Bo Zhang,
You can use BCS for basic planning with multiple budget versions. PBF is used for more complex budget development processes. This includes:
1. Budget requests from departments with multi-stage approval
2. Personnel expenditure planning with integration to HCM data
3. Planning of individual commodity costs based on standard rates
4. Planning of decisions with incremental costs and revenues
Also PBF is based on BI and offers what-if analysis and planning functions in a user-friendly interface.
Thanks
Shyam