On p25 in Keller's Learn to Read Latin:
As is explained in Section 7, the fourth principal part of a verb is usually the perfect passive participle ofthat verb. in this book, two different endings of the perfect passive participle - -us and -um - are used in the vocabulary lists beginning each chapter. Verbs that are transitive that express an action that is directly exerted on a person or thing- - have a fourth principal part ending in -us. Verbs that are intransitive - that express an action that is not directly exerted on a person or thing - have a fourth principal part ending in -um. For more on the distinction between transitive and intransitive verbs, see Section 11.
Certain intransitive verbs have no perfect passive participle. For some intransitive verbs a future active participle, whose ending is -urns. is supplied as the fourth principal part. This is true, for example, for the irregular intransitive verb sum, esse, fui, futurus. The fourth principal part, futurus, is a future active participle.
Certain verbs, both transitive and intransitive, have no fourth principal part at all. When memorizing and reciting aloud such verbs, say "blank" for the missing fourth principal part.
In the first sentence highlighted in bold, what is the fourth principal part, said to be ended in -um, for an intransitive verb? Is it also the perfect passive participle of the intransitive verb?
Note that the second sentence highlighted in hold, "Certain intransitive verbs have no perfect passive participle", is mentioned only after the first highlighted sentence, so I guess it doesn't apply to the first highlighted sentence, i.e. in the first highlighted sentence, an intransitive verb has a perfect passive participle?