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You can't remove an activity type for a project if there is a least one work log with this activity type. This applies only for work logs which are NOT accounted.
This also has to be considered when switching from using all activity types to using only selected activity types, in this case you will have to add all activity types that already have been used. You get a respective error message if you are trying to remove (or not to add) an activity type with existing work logs. Depending on the number of work logs affected, you could run a report that delivers these work logs and might change the activity type (see Reporting).
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