What are Unlimited Rules?
When configuring a sync between two apps, start with two rules by default. Depending on the app, you can choose to only sync a subset of your contacts by filtering them based on tags, groups, lists,... In a rule, you can also include actions that PieSync should perform, such as adding a tag or (un)subscribing someone to a list.
You can add more rules: this feature is called Unlimited Rules and is available from the Medium plan onward. For these plans, the number of rules you are allowed to set up in one connection is unlimited and dependent on the number of subsets you have and want to keep in sync between two apps.