Pro Tools preferences are either
global (system-wide) or
local (for the current session only).
Global preferences are applied to all sessions. These preferences become the default preferences for any existing sessions that you subsequently open or any new sessions that are created.
Local preferences are applied to the current session (if one is open). Local preferences are not applied to older sessions that are subsequently opened.
New sessions use global preference settings, plus the most recent local preference settings. The most recent local settings refer to the last open session (unless subsequent changes were made in the Preferences from the application window).