If you want to decide what data type a certain field has, this is the feature that can give you this.
The issue could be this. You have a Excel data source with some sheets that contains a field with only numbers. Even though it actually is a Item Id, it gets treated as a numeric value.
The issue could also be this. You have a data source connection to a database with a datetime field that contains both nulls and dates lower than 1753-01-01.
In both cases the overwrite feature could help. You can set up a general rule, or a specific rule.
Set up a general rule
Start by opening the data source settings menu and go to the Override Data Type pane.
As standard all data sources already have one rule set up. It is that all unknown data types will be converted to varchar(50)
To create a new rule you start by pressing Add. If you want it to be towards a specific Schema, table or Field then set the selector to one that fits the rule you want to apply.
After this you choose what data type you want to convert to.
Say you want to convert all varchar fields to nvarchar(50). It would look like this.
Remember that the schema needs to be the one that are in the data source, not the one you created in this system.
After this you synchronize and the changes should be seen.
Set up a specific rule
Say your issue is more specific. You have added a new excel sheet and it translates a integer field into a numeric.
Like this excel sheet
First step is to find the field you want to change in the data source. Right click on the field and press Override Data Type.
Find the data type you want it to become.
Set the type of field you want, like unicode if it is a text type.
The next step is to synchronize with the data source.
You will then be notified about the changes you made.
If you look at the Data type overrides pane again, you now have a new rule.