The only official guideline for "Minecraft" usernames is that the name must be available. The username must not already be in use or have been used in the last 37 days. That is because a username is on hold for 37 days after it is changed by a user.
Usernames are necessary in "Minecraft" when users play multiplayer games with other users online. These multiplayer games are hosted by local servers, and the host has the right to ban users with rude, racist or otherwise offensive usernames. Mojang, the developer of "Minecraft," also changes exceptionally offensive usernames to names that are more family-friendly.