While we'll always add new data to blank fields on each activist when you upload, when a field is already filled we will by default we favor data that already exists in our database (such as activist-inputted data) over new, uploaded data. So if an activist has given us a ZIP/postal code or already has a value for a certain custom field and your upload has a different ZIP/postal code or value, we'll favor the one we already have and not overwrite it with your new data when you upload.
You can overrule this preference by checking the overwrite box when you upload a file, if you have the "Advanced Upload" permission for the group. We will prefer the data in your file in this case, and overwrite any data a matching activist has entered. Be careful with this option, it cannot be undone.
Note: We will not remove any data, even if that field is blank in your upload, no matter if the overwrite checkbox is checked or not, unless you check the clear blank values option as explained here.
Note: If the custom field you are uploading into has validation and the data you are uploading for that activist does not match the validation expression, the data from the upload for that activist will be skipped and not added to the activist's record.