var mainSubDocumentText = richEdit.document.getText();

var selectedText = richEdit.selection.activeSubDocument.getText(richEdit.selection.intervals[0]);

var mainSubDocumentLength = richEdit.document.length;
// or 
var mainSubDocumentLength = richEdit.document.subDocuments.main.length;

var subDocument = richEdit.selection.activeSubDocument;
var position = richEdit.selection.active;
subDocument.insertText(position, 'text');

var subDocument = richEdit.selection.activeSubDocument;
var position = subDocument.length - 1;
subDocument.insertText(position, 'text');

var subDocument = richEdit.document.subDocuments.main;
subDocument.insertText(subDocument.length - 1, 'text'); 
// or
richEdit.document.insertText(richEdit.document.length - 1, 'text');

var createHeaderIfNotExist = true;
var section = richEdit.document.sections.getByIndex(0);
var subDocument = section.getHeader(DevExpress.RichEdit.HeaderFooterType.Primary, createHeaderIfNotExist);
var position = 0;
subDocument.insertText(position, 'text');

var subDocument = richEdit.selection.activeSubDocument;
subDocument.deleteText(subDocument.interval);

var subDocument = richEdit.selection.activeSubDocument;
var position = richEdit.selection.active;
var characterProperties = {
    bold: true,
    fontName: richEdit.document.fonts.getByIndex(0).name,
    highlightColor: "ffff00",
};

richEdit.history.beginTransaction();
richEdit.beginUpdate();
var interval = subDocument.insertText(position, "text");
subDocument.setCharacterProperties(interval, characterProperties);
richEdit.endUpdate();
richEdit.history.endTransaction();

var subDocument = richEdit.selection.activeSubDocument;
var position = richEdit.selection.active;
var templateText = '[Type your text here]';

richEdit.beginUpdate();
richEdit.history.beginTransaction();

position = subDocument.insertParagraph(position).interval.end;
position = subDocument.insertParagraph(position).interval.end;
position = subDocument.insertText(position, 'Dear Mr Stanley,').end;
position = subDocument.insertParagraph(position).interval.end;
var tmpTextInterval = subDocument.insertText(position, templateText);
position = tmpTextInterval.end;
position = subDocument.insertParagraph(position).interval.end;

richEdit.endUpdate();
richEdit.history.endTransaction();

richEdit.selection.setSelection(tmpTextInterval);
richEdit.focus();