The initial batch of controls on the Calculation tab deal with calculation options that affect all projects opened within Microsoft Project.
This option makes an automatic connection between the status of the task and the status of the resource assignments on that task. With this box checked, any edit to the status of an assignment on a task will automatically adjust the status of the task itself and edits to the status of the task will automatically adjust the status of the assignments on the task.
If you uncheck this option, you need to edit the task status fields (% Complete, Actual Start, Actual Work, and so on) as well as the corresponding fields for the assignments to fully give the status of the task and its assignments. Newcomers to Project should leave this option checked until they are fully comfortable with the concept of the separation of these two sets of status fields.
This option controls how Project distributes edits to Task % Complete. With this box unchecked, the % Complete is distributed from the actual start of the task to the actual finish date. With the option checked, the % Complete is distributed from the actual start to the project status date (or to the current date if no status date is set). This option is unchecked by default, and this is the best setting for new users of Project. Checking this option is a good idea for users who make use of Earned Value tracking, and who set the status date of the project to control how Project calculates Earned Value metrics.