Urgency

• https://www.debian.org/doc/manuals/developers-reference/pkgs.html
• https://www.debian.org/doc/debian-policy/#s-f-urgency
• https://www.debian.org/devel/testing
• https://lists.debian.org/debian-mentors/2008/02/msg00567.html

IIRC valid options are:

• low
• medium
• high
• emergency (I think)
• critical (I think)

A default of low is sensible.