Many approaches suggest themselves.

A) A boolean attribute, $Important, flags important notes.

     AgentQuery: $Important==true

B) Add "Important" to $Tags

    AgentQuery: $Tags.contains("Important")

    This is a bit more trouble, but also more flexible.  You could, for example, also have notes that are VeryImportant, MildlyImportant, etc...

C) A numeric attribute $Importance rates the importance of a note on (say) a scale of 0 to 9.

   AgentQuery: $Importance>7

D) A string attribute $Significance, characterizes the significance of the note using a predefined set of keywords: [slight,modest,important,crucial]

   AgentQuery: $Significance=="important" || $Significance=="crucial"

E) Assign a badge to the important items

F) Use $BorderColor to gauge the importance of items, using $BorderColor 1 for unimportant things and 9 for important things.

G) Use $NameBold to signal important things.

There are lots more....

$Badge is a String attribute so work on the assumption that matches are case sensitive, i.e. it will match 'BadgeX' but not 'badgex'.

If you make an agent with the query:

  $Badge=="BadgeX"

It will collect an alias to all notes using the 'BadgeX' badge.

Does that help?