Every key must correspond to a key in the custom message data that you have set. It is not necessary for all messages to have these keys.

Each value must be one of the following:

A string, equal to "exists" or "!exists"

A 2-element array of [operator, operand] structure. The operand must be either a string or an array of strings (for the oneOf operators). Valid operators are: "==", "!=", "oneOf", and "!oneOf".

Examples, assuming you have set a category custom field on your messages:

// Only show messages that have no category set:
{ custom: { category: "!exists" } }
// Only show messages of that have the category "shoes"
{ custom: { category: ["==", "shoes"] } }
// Only show messages that have the 'topic' either "inquiry" or "reservation"
{ custom: { topic: ["oneOf", ["inquiry", "reservation"] ] } }
// Only show messages about shoes that are marked visible.
// this assumes you also have a custom field called visibility
{ custom: { category: ["==", "shoes"], visibility: ["==", "visible" ] } }