For the employee it is total FICA except for wages paid by the same employer. If a single employer has over withheld the FICA, the employee has to get the excess taxes withheld from the employer. The employer is not necessarily determined by the same EFIN.
See IRS Pub 17, 505, and 553 for more information
For the employer it is by employee and if the employer over withholds the employer reimburses the employee and files form 843 to claim the excess FICA withheld.
The excess FICA w/h on a variety of W-2s should show as an overpayment of the FICA. It then gets applied the fit and any overpayment gets refunded to the taxpayer by the IRS.
Each employer is resposible for w/h FICA up to the max. However, the taxpayer only has to pay up to the max w/h once.