On a paid plan, the font files you design and download are yours. You may install them, use them in any project, give them away, license them to others and sell them, including selling the fonts themselves. One Custom Font covers the one font family your purchase is used on. The editor only downloads fonts that have been changed enough from Letterhead's own typefaces and styles to be your own design.
On the Free plan, fonts you download are for personal, noncommercial use only. To use them commercially, upgrade before you publish or sell anything made with them; the commercial license then covers that font.
Credit. Every font you download carries a short note in its file information: “Made with Letterhead.” If you sell or give away a font made with Letterhead, keep that note in the font files, and include the line “Made with Letterhead” in the license you give the people who get it. You do not need to put it in your product descriptions or sales pages, in your designs, or anywhere the font is used.
Letterhead Editorial is based on Spectral by Production Type, which is released under the SIL Open Font License 1.1. Fonts made from Letterhead Editorial stay under that license, whatever your plan: you may use and share them freely, including in commercial work, but you may not sell the font files on their own, and you must keep the copyright and license notice in the files. The app marks this typeface with an Open license badge.
Copyright and design protection for typefaces differ from country to country. Owning your font files does not mean anyone has registered rights in the letter shapes. If a font matters to your business, consider your own trademark for its name.