We abstract a message by replacing information carrying characters with a distinguished character from the class.
Expected letters are replaced with "A" or "AA".
Integer numbers become one or more "9".
Maidenhead locations show as "ZZ99".
The symbols "+" and "-" show as "-".
http://ft8.ward.asia.wiki.org/assets/pages/spark-spots/abstract.html HEIGHT 350
More about callsigns. wikipedia 
More about maidenhead locations. wikipedia
More about FT8 decoding. wsjtx manual
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47 CFR 97.119(c): One or more indicators may be included with the call sign. Each indicator must be separated from the call sign by the slant mark (/) or by any suitable word that denotes the slant mark. If an indicator is self-assigned, it must be included before, after, or both before and after, the call sign. No self-assigned indicator may conflict with any other indicator specified by the FCC Rules or with any prefix assigned to another country. mastodon