Different types of text-based objects are not interchangeable, see Text types. Staff Text object and System Text object are used for purposes not covered by other text types only. Including:
"Expression" is a Staff Text object using "Style for text inside Expression text". See "Style" section below.
Both of them attaches to a note or rest in a Musescore Instrument.
Both of them can be set-up to have effect on audio created by Musescore's Synthesizer, a Staff Text affects only the playback of that Musescore Instrument is affected, a System Text affects all Musescore Instrument on a score.
Staff Texts are hidden along with "Hide empty staves within systems" option (and the sub-option "Don't hide empty staves in first system") under "Style" window : "Score" , see Layout and formatting.
A Staff Text only appears in the Musescore Part(s) that features the Musescore Instrument. A System Text appears on all Musescore Parts.
New System Text is positioned above the top staff of each system (layout concept).
To add one onto a score:
New System Text is positioned above the top staff of each system (layout concept). If you need a similar indication on a lower staff, add it to that staff using Staff Text.
Fomatting of a Staff Text object or System Text object on a score is edited in the Inspector, covered in Text basics chapter.
The playback settings are edited in "Staff Text properties" and "System Text properties" window
Staff Text contains settings that affect playback. Only the section after the attached note or rest is affect. When a Staff Text is duplicated, or reused through customized palette, the playback settings are reused.
The playback settings can be accessed in two ways:
The four tabs on top are:
Works like "Staff Text properties", but only has "Capo Settings" and "MIDI Action" tabs.
"Style for text inside Staff Text" is special, see Text styles and properties.
Values of the "Style for Staff Text" can be edited in Format→Style→Staff Text.
Values of the "Style for text inside Staff Text" can be edited in Format→Style→Text styles→Staff
Values of the "Style for text inside Expression" can be edited in Format→Style→Text styles→Expression
Values of the "Style for text inside System Text" can be edited in Format→Style→Text styles→System