Boot from SD card & eMMC

In my Beaglebone black board, I have loaded eMMC with an image. If i power on, then BBB is booted from eMMC.

If i insert SD card with a different image, and power on the BBB, it starts to boot from SD card. I did not press boot button here.

My thought/expectation is like if i
a.Press the boot button and power on, BBB has to boot from SD card.
b.Don’t press the boot button and power on, BBB has to boot from eMMC.

Please correct me if my understanding here. How to achieve the mentioned options a,b with BBB.

By default “u-boot” on the eMMC will scan the microSD for a bootable image (it looks for /uEnv.txt and /boot/uEnv.txt trigger files and if u-boot finds them it’ll boot it)…

Regards,