Why do I need "field:" in my attribute declaration "[field:NonSerialized]"?

I can't find "field" listed as a C# keyword anywhere. Does anyone know the background on this?

What is NonSerialized in C#?

The NonSerialized attribute marks a variable to not be serialized.

How do you make a field not serializable in C#?

When using the BinaryFormatter or SoapFormatter classes to serialize an object, use the NonSerializedAttribute attribute to prevent a field from being serialized. For example, you can use this attribute to prevent the serialization of sensitive data.

This is necessary, for example, if you are marking an event as non-serializable. It specifies the target that the attribute applies to.

It is part of the attribute target syntax. From the specification:

attribute-target:
    field
    event
    method
    param
    property
    return
    type

See also the documentation for NonSerializedAttribute:

To apply the NonSerializedAttribute class to an event, set the attribute location to field, as shown in the following C# code.

[field:NonSerializedAttribute()]

public event ChangedEventHandler Changed;