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Box 7 on the W2 form. How to calculate the value?

I've been asked to calculate the amount for Box 7 on the W2 form for end of year filling.

The best description I've found online is:

Box 7 is the total of  reported tips subject to Social Security tax. It does not include allocated tips.

I'm not sure what amount of the reported tips are subject to Social Security Tax. Also, what is the definition of an allocated tip ?