The way emails are assigned to a specific tab depends on multiple criteria, including but not limited to the following:
- Gmail uses a variety of signals to classify messages into the different categories.
- Users are in control of their inboxes - they can disable some or all tabs, they can choose to move particular messages or senders into a particular category manually.
- Gmail learns from user corrections, so over time any misclassifications, as users who receive the mail perceive them, will get automatically put into the right category.
- It is recommended for commercial mail senders to split promotional mail from notifications/transactional mail - put those in different messages and send then from different email addresses.