Note: debug mode is not something Bandai intended for customers to use. To place a Tamagotchi into debug mode, it is necessary to take the Tamagotchi apart. I don't recommend you try this. If you decide to try it and something goes wrong, and you end up with a broken Tamagotchi, don't blame us.
When a Tamagotchi is placed into debug mode, all information about current characters, points, purchased items, family history, etc are erased.
Generally, when a Tamagotchi is in debug mode, you get to select the type of character to raise. Also, when debug is enabled, you can place it into fast mode, so time advances at a faster-than-normal rate (for example, 1 minute on your Tamagotchi's clock takes only 1 second of real time.)
There is a site here that explains a bit about debug mode on an early Connection (V1 or maybe V2):
https://www.pixelmood.com/tamagotchi_connection_debugging.htm
There is a topic about debugging a V5 on Tama-Zone. Here's the link:
https://tama-zone.com/index.php?showtopic=17671