Official comment
There are two basic types of Rules in ONTRAPORT, those that are triggered at a particular point in time and those that wait for another event to happen and act regardless of the date or time.
Step Sequences have Rules that act only when the contact is on that step. If the Rule step is set to act on the third day, it will fire on third day only. It checks against the current situation at that time, and is inactive thereafter. If the Rule is looking for a particular Tag, and the contact does not have that Tag on the third day when the Rule checks, it does not complete the action, even if the Tag is added a day later.
Date Sequences also have Rules that act only when the date / time for that Rule comes around. Before or after that date and time the Rule is not active. If the Rule is set to check for a Tag at 10:00 AM on the 14th, and the Tag does not exist at that time, the Rule will not fire and complete the action. Not even if the Tag is added later, at 10:15 AM.
Rules created in Contacts > Rules (global Rules) are different. They act whenever the condition changes and meets the requirements. A Rule checking for the presence of the Tag will fire if the Tag is added to the contact at any time after the Rule is created.