The meanings of "unless" and "unless otherwise" are very similar and often the same. The phrase "unless otherwise" is usually used in giving instructions to make them more clear.
Here are some equivalent phrases using unless and unless otherwise:
Unless Otherwise
You may smoke unless otherwise instructed.
Unless otherwise instructed, please remain seated.
Unless you are told otherwise, you may take photographs.
Unless
You may smoke unless instructed not to smoke.
Unless instructed to stand, please remain seated.
Unless you are told not to, you may take photographs.