This term is used for television and radio advertising. When you’re buying advertising, BTA stands for Best Time Available. Do not confuse BTA with ROS. ROS stands for RUN OF SCHEDULE.
While BTA means your ad will run during the BEST time available, ROS means your ads will run THROUGHOUT the schedule.
With BTA, if there are spots available during the morning drive, and morning drive is the best time on the station, then in theory, your ad will be placed in those spots.
With ROS, if there are 15 spots available in the morning drive slot, your 12 daily spot schedule will have a spot playing approximately every other hour. So your spot may be placed once or twice in morning drive, it will also air after midnight as well.