The assumptive mood is an epistemic

Epistemic modality

Epistemic modality is a sub-type of linguistic modality that deals with a speaker's evaluation/judgment of, degree of confidence in, or belief of the knowledge upon which a proposition is based. In other words, epistemic modality refers to the way speakers communicate their doubts, certainties, and...

 grammatical mood

Grammatical mood

In linguistics, grammatical mood is a grammatical feature of verbs, used to signal modality. That is, it is the use of verbal inflections that allow speakers to express their attitude toward what they are saying...

 found in some languages, which indicates that the statement is assumed to be true, because it usually is under similar circumstances, although there may not be any specific evidence that it is true in this particular case. An English

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

example (even though assumptive mood is not specially marked in English), would be "That must be my mother. (She always comes at this time.)"