Since this communication is often necessary, it is important to take into account that each bilateral agreement is different and may set different coverage and contribution criteria, and that the very existence of a bilateral agreement may result in a greater loss for the expert seconded abroad with regard to future tax benefits. Data protection law requires us to inform you that we have the right to collect this information in accordance with section 233 of the Social Security Act. Although it is not mandatory for you to provide information from the Social Security Administration (SSA), a certificate of coverage can only be issued if an application has been received. The information is necessary to enable the SSA to determine whether the work should only be covered by the U.S. social security system, in accordance with an international agreement. Without the certificate, work can be taxed under both the U.S. and foreign social security system. A separate agreement, called a totalization agreement, helps U.S. expats in Spain not pay Social Security taxes to the U.S. and Spanish governments.