It’s often useful to store status information in a custom list such as an approval code which should never be directly manipulated by end users. This is a common business scenario. I have been working on a project this year that generates various status and reminder dates via SharePoint Designer workflows that then drive KPIs and generally support time-critical business processes.
Use a site content type / column to achieve this effect.
Create a site content type with the status column as shown:
Note the "Status (for Demo)" field at the bottom.
Next, click on the column name to access the column’s properties. Select "Hidden (Will not appear in forms)" in the Column Settings section as shown:
This removes the field from the standard edit/update forms. However, it is still available in views, KPIs, the object model and anywhere else you want to use it, including SPD workflow.