This refers to the audio codec (Dolby Digital). The "5.1" means it supports a full surround sound setup (five speakers and one subwoofer). "pxac3" / "51d":
Thomas Reynolds (Jon Voight), a corrupt NSA official. enemigo publico 1998 bluray 1080 pxac3 51d
AC3 5.1 Surround Sound – Dynamic audio that captures every explosion and high-tech surveillance beep. This refers to the audio codec (Dolby Digital)
John Woo's signature style is on full display in "Enemy Public," with a non-stop barrage of explosive set pieces, intense hand-to-hand combat, and death-defying stunts. The film's kinetic energy is infectious, propelling viewers into the midst of the action and refusing to let up. intense hand-to-hand combat
The metadata in your query describes a specific digital backup of the film: : Indicates a high-definition resolution of