How does the Fixed Dates (FD) constraint affect my project schedule?
Project Central uses constraint type rules, along with work durations and dependencies, to determine the scheduled start and scheduled end dates of work items in your plan. With FD selected as the constraint type, the schedule will not be calculated, it will be the entered dates. This article describes how the FD constraint works.
Note: Because fixed dates impede the ability of the scheduling engine to properly schedule the project, it is recommended that you use this constraint type only when absolutely necessary.