Sure. We have some documentation about this. See this part:
Prematch determines whether the field should be used to match objects between Salesforce and WordPress, regardless of the direction. If it is checked, the plugin always will see objects that share a value in that pair to be matched.
A Salesforce Key is a flag in Salesforce that determines whether a field is another system’s external ID. Salesforce uses this to keep data from being overwritten.
A checked Prematch (when saving data in WordPress or Salesforce) or Salesforce Key (only when saving data from WordPress to Salesforce) will cause the plugin to try to “upsert” the data to its destination. This means it checks to see if there is a match before creating a new record.
Here’s an example of why you would use prematch:
1. A Contact exists in Salesforce with a certain email address.
2. A user creates a new account in WordPress, using the same email address.
3. If the email field were selected as a prematch field, these would be matched to each other. If the email field were not a prematch, a new Contact would’ve been created with the same email address as the existing one.
Salesforce key is a Salesforce field that means a field is an ID field for a non-Salesforce system. It’s not really used in this plugin, but it can be helpful for understanding how your Salesforce structure works, and that’s why we display it. For example, if you had a Salesforce Contact that was mapped to a MailChimp list subscriber, the MC list subscriber ID should be flagged in Salesforce as a key field. The plugin would check that box as well, in that case, but it doesn’t affect how WordPress works.