If the actual date of a milestone is linked with a database column, the date is derived from this column and must not be entered manually. For example, if the date is linked with m_po_headers.approved_date, the actual date is automatically set when the purchase order is approved.
indicates that the actual date is linked with a database column, and 
indicates, that the actual date is no longer linked and the date can be entered manually.
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Select a milestone, and click
in the Actual cell.
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A message appears, click Yes.
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Click
in the Actual cell and select a date.
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Another message appears, if the forecasted dates shall be recalculated.
To link the actual date again with a database column, click and confirm the message with OK.