What's meaning of the expression eventProperty::target.properties(email) ?

[edfeff]

Hi!
Some json definition file(Action,Rule,...) has confused me.
For example,

"actions": [
    {
      "parameterValues": {
        "mergeProfilePropertyValue": "eventProperty::target.properties(email)",
        "mergeProfilePropertyName": "mergeIdentifier"
      },
      "type": "mergeProfilesOnPropertyAction"
    },
]

What's meaning of the expression eventProperty::target.properties(email) ?
Are these expressions have specifications or documentation?

Thanks

[sergehuber]

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