Difference Between "Alternative Password" and "Backup Password?

I set up the fingerprint lock on my new S6 yesterday. My backup password is a combo of letters and numerals. Today I'm trying to install Fingerprint Security app, and it's asking me for an "alternative password" to open the app. :-/ I have no clue, and FS doesn't give me the option of entering any letters. Help, please?

NM. I contacted the dev. for answer.

I would love to know what that is all about. When I got locked out, I was presented with that option, an all number alternative, then it didn't recognize it anyway.

I didn't remember setting an all-numbers password, so it scared me a bit. When I emailed the developer of the app, he said the default for the app is 1234. I asked how I would know that if I hadn't emailed him. He said he didn't know, and that he needed to change that. :-/ OK, but what about all the thousands of people using this app? How did they know the default was 1234? IDGI.