I saved a SimpleText document in TextEdit and can't open it in SimpleText
You can open SimpleText documents in TextEdit. However, if you save the document in TextEdit, it is automatically converted to an RTF file.
To make sure SimpleText can open the document, you can save it as a plain text file. Choose Format > Make Plain Text and then save your document. However, when you convert a document to plain text, all formatting (such as bold text) is removed.
If you want to make changes to a SimpleText document and retain the original file type, open it using the SimpleText application in the Classic environment.