United Arab Emirates Flags

Merchant ships may fly the alternative civil ensign, a red flag with the national flag in the canton. All Emirates use the federal flag interchangeably as the flag of the emirate.

Each of the seven emirates within the United Arab Emirates originally had a common red plain flag as each emirate's banner, the red banner represents the descension and allegiance to the prophet Muhammad. In 1820, six out of the seven emirates signed the General treaty agreement with the British Empire which compelled them to be under the British Protectorate rule and protection in the region. A white segment was enforced to be added to the hoist of each emirate's flag.

Fujairah was the only emirate which did not sign the general treaty in 1820 with the British protectorate and therefore continued to use its plain red flag. Wikipedia