src/Adapter/Concerns/GuardsFields.php
protected function isFillable($field, $record)
    {
        /** If the field is listed in the fillable fields, it can be filled. */
        if (in_array($field, $fillable = $this->getFillable($record))) {
            return true;
        }

        /** If the field is listed in the guarded fields, it cannot be filled. */
        if ($this->isGuarded($field, $record)) {
            return false;
        }

        /** Otherwise we can fill if everything is fillable. */
        return empty($fillable);
    }
protected function isFillable($field, $record)
    {
        /** If the field is listed in the fillable fields, it can be filled. */
        if (in_array($field, $fillable = $this->getFillable($record))) {
            return true;
        }

        /** If the field is listed in the guarded fields, it cannot be filled. */
        if ($this->isGuarded($field, $record)) {
            return false;
        }

        /** Otherwise we can fill if everything is fillable. */
        return empty($fillable);
    }
Is the JSON API field allowed to be filled into the supplied record?

@param $field
@param $record
@return bool