I don’t understand what you mean by square characters: can you make that more clear?
There are text editors you can use I guess for the parallel aspect of what seems to be a measure-wise notation IIUC, i.e. those that offer block-mode editing such as Win-EDT on Windows.
There’s a tree structure in nearly all music, written or played: parts are performed in parallel, each of them made of voices. The latter usually get grouped into staves for reading and organisation commodity, and repeats, da capos and codas add more structure to that. An organ music score is an example of such a tree.
All text notations used to represent trees have a difficult problem. MusicXML is not meant to be used by composers or music aficionados, it is an exchange format designed for use by computer applications. The order of the various markups such as <part-list/> and <part-list/> is defined by a DTD.
In the example below, the <note/> contains the sharp <accidental/>, but the <p /> dynamic occurs before it. It could have been placed inside the <note/> too, though. Such design choices were not made at random, there are reasons behind them.
<direction placement="below">
<direction-type>
<dynamics>
<p />
</dynamics>
</direction-type>
</direction>
<note>
<pitch>
<step>C</step>
<alter>1</alter>
<octave>4</octave>
</pitch>
<duration>16</duration>
<voice>1</voice>
<type>half</type>
<accidental>sharp</accidental>
</note>
In this other example, there’s a partgroup containing two parts, one for each flute, with respective parts « 1 » and « 2 » sharing a single staff as is often the case in orchestral scores.
</part-group>
<score-part id="P2">
<part-name>Flutes</part-name>
<part-abbreviation>Fl.</part-abbreviation>
<score-instrument id="P2-I19">
<instrument-name>Fl.</instrument-name>
</score-instrument>
<midi-instrument id="P2-I19">
<midi-channel>2</midi-channel>
<midi-program>74</midi-program>
</midi-instrument>
</score-part>
<part-group number="2" type="stop"/>
<part-group number="2" type="start">
<group-name>1
2</group-name>
<group-barline>yes</group-barline>
</part-group>
And the horns sections is a sub-partgroup in this score, with 4 voices grouped into two staves. MusicXML precisely is weird in this area BTW: it does not represent a staff group (tree of groups) as a tree, by allows for « intelaced groups », which is arguable:
How do you represent such complex structures with Premusic?
JM