# Notes and Constraints

The following are basic restrictions on the tag usage:

- Each resource supports up to 20 key-value pairs.
- For each resource, each tag key must be unique, and each tag key can have only one tag value. If the tag value you add is the same as an existing one on the resource, the new value overwrites the old value.
- Each user can create up to 500 predefined tags.
- If the created predefined tag is the same as an existing predefined tag, the existing predefined tag is overwritten. If only keys are the same and values are different, the new predefined tag is created.
- You can enter a maximum of 36 and 43 characters for Key and Value, respectively. Both Key and Value can contain only digits, letters, hyphens (-), at signs (@), and underscores (_).

Parent topic: Product Profile