Enums are used in the API to define different values, here are the most common ones.

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Status

Declaring the status of various objects, most notably cards 
-2: Junked
-1: [only used internally]
0: Pending cards (inbox)
1: Unkept cards
2: Kept cards

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Visibility

Set whether a card is Priority (shown in the Outline), Visible (only shown in the Noteboard), or Invisible (by default hidden in the Noteboard and search results): 
enum Visibility {
  PRIORITY = 1
  VISIBLE = 0
  INVISIBLE = -1
}

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Roles & Perms

Set different roles and corresponding permissions on a card: 
enum Role {
  INHERIT = -1,
  NONE = 0,
  READER = 1318,
  CONTRIBUTOR = 1382,
  EDITOR = 1398,
  MODERATOR = 4094,
  AUTHOR = 8190,
}

enum Perm {
  VIEW_COMMENTS = 2,
  ADD_COMMENT = 4,
  MANAGE_COMMENTS = 8,
  EDIT_DATA = 16,
  ADD_CHILDREN = 32,
  PUBLISH_CHILDREN = 64,
  MANAGE_PARENTS = 128,
  CREATE_SHARE_CODES = 256,
  MANAGE_SHARE_CODES = 512,
  VIEW_MEMBERS = 1024,
  MANAGE_MEMBERS = 2048,
  DELETE_FOR_EVERYONE = 4096,
}

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View Display Type

Set the Noteboard to be a certain view: 
enum ViewDisplayType {
  LIST = 1
  BROADSHEET = 2
  GRAPH = 4
}