Phrase versus Slurs

There's no alternative AFAIK

In wind instruments, a slur that is longer than a musician can be reasonable expected to play in one breath must be a phrase mark. It also tells the musician to play very legato (minimum tonging or airflow interruption possible). The symbols are identical but their interpretations are slightly different. The distinction between a slur and a phrase mark can be quite fuzzy.

If you don't like the direction of a slur, then select it and press X and it will flip.

MuseScore does indeed treat the phrase mark and the slur as one and the same. They are both called a "slur" in the Inspector, and there is no difference in playback between them.

In reality, the difference between the two is only a matter of degree. Both indicate that the marked notes are to be played in a connected way. The only difference between legato and slur is that the slurred notes are perhaps even more connected than simpy legato notes.

"A 'phrase mark' . . . may cover a longer passage . . ." is one key to distinguishing the two: so for example, in the attached example, the mark spanning all four measures is clearly the "phrase mark" and the mark spanning the two notes in measure 1 and the mark spanning measure 2 are "slurs." When you only have one or the other, however, they are ambiguous. If a mark only spans two notes, it is clearly a slur. If a mark spans a range of notes without there being also a phrase mark spanning a larger range, there is no way to tell from the mark itself whether it is meant as a slur or a legato. But once again, "may cover a longer passage" in your quote is key: the musical context determines whether it is a slur or a legato; i.e., if the span is an entire phrase, the mark is meant as legato - if the span is a shorter range within a phrase, it is a slur.

But to the crux of your question: yes, you have to use the "slur" function since MuseScore does not have a separate "phrase mark" function. But you can control whether the "slur" appears above or below the notes and how far above or below it appears by selecting it and changing the "direction" and "vertical offest" parameters in the Inspector. This, in fact may be a further way to make clear the distinction you want between a "slur" and a "phrase mark" - editors tend to print phrase marks higher above the notes than slurs.

While there can indeed be a subtle musical distinction in meaning here, there is no such difference in standard musical notation. The exact same symbol is always used for both. Unlike ties, which although they look similar, are actually subtly difference in appearance, and that's why they are provided as a separate thing in MuseScore. But there quite simply is no separate concept of a "phrase marking" in standard musical notation. The one universally used way of indicating phrases is using slurs.

Realistically, phrase markings as such are not commonly used in music for wind instruments precisely for this reason. Pianists know that all slurs are really phrase markings because the instrument has no concept of a slur. String players can often guess from context that a slur is really a phrase marking (if it cover more notes than can physically be taken on one bow, which often isn't very many). But wind players won't have any way of knowing you aren't telling them to slur, at least not when talking about a phrase of only two measures (for a 16-measure phrase, they might get the hint). I guess you could add individual articulations under the phrase marking, but no guarantees the players would understand you - most would probably be confused.

To flip slurs (or other elements) up/down, press "X".