What happens when a contact in my file partially corresponds with an existing contact in Spotler?

This depends on the settings of the deduplication process. Deduplication is done by default based on an email address. This means that when a contact in your list has the same email address as the existing contact in Spotler, the details of the existing contact in Spotler will be supplemented and /or possibly overwritten with the details in your file. Therefore, it is always good to check which system or file contains the most up-to-date details, so you do not overwrite new details with old details.