You're seeing just the function set_new_attribute, go back to Ash.Resource.Change.Builtins module for more information.
Link to this function

set_new_attribute(attribute, value)

View Source

Sets the attribute to the value provided if the attribtue is not already being changed.

If a zero argument function is provided, it is called to determine the value.

If a tuple of {:arg, :argument_name} is provided, the value will be read from the argument if supplied. If the argument is not supplied then nothing happens.