Extra is usually used as an adjective, or a noun:-
| Adjective: |
| Used to describe something added to an existing or usual amount or number. |
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| Noun: |
| An item in addition to what is usual or strictly necessary. |
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For example: We set an extra place in case she brought her new boyfriend.
Surplus can be a noun or an adjective:-
| Noun: |
| An amount of something left over when requirements have been met; an excess of production or supply over demand. |
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| Adjective: |
| More than needed or used. |
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For example: Luckily they were able to return the surplus.
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