大般涅槃經義記 5.318

5.318

辯第二中，先問、後釋。如緣父母、兄弟、眷屬與樂心易，故名自在。所緣狹少，故非無量。既非無量，云何上言是無量心有四種乎？蓋乃是其無量中分，故名無量。如一比丘，眾僧中分，亦名眾僧，此亦如是。

In distinguishing the second, he first asks and then explains. For instance, when taking parents, siblings, and relatives as objects, the mind that bestows happiness arises easily, and therefore it is called "sovereign." The object taken is narrow and few, and therefore it is not immeasurable. Since it is not immeasurable, why does it say above that this immeasurable mind has four kinds? It is simply a part within the immeasurable, and therefore it is called immeasurable. Just as a single monk is a part within the Saṃgha and is also called the Saṃgha, this is also like that.