Fixed 13.7V rail — what it's for
The STMicroelectronics PB137ACV is a single-output positive linear regulator that delivers a fixed 13.7 V at up to 1 A. The TO-220-3 through-hole package suits point-of-load regulation on a power board or battery charger PCB.
58 dB PSRR — ripple rejection on the rectified line
The 58 dB power-supply rejection ratio at 120 Hz corresponds to the fundamental ripple from a full-wave bridge rectifier. For higher-frequency noise the PSRR rolls off; a post-filter cap or a small series inductor helps if the load is sensitive.