You can find a list of reserved keywords here: http://techcommunity.softwareag.com/ecosystem/documentation/natural/nat824mf/pg/pg_keyw.htm
Reserved keywords don’t have to have a meaning by themselves but the context in which they are used would lend itself to what it means in that context. Natural doesn’t want you to define variables that could be confused with placement of various words as that can lead the compiler to try to decide an ambiguous string that it cannot properly parse. Though as a side note, participants on SAG-L have shared some rather ambiguous-looking code that the compiler had no trouble interpreting.
ALARM for example is included under REINPUT syntax to indicate that the machine should “beep” at the user while displaying any reinput messages, and if specified, reposition the cursor using MARK.
See documentation reference and specifically concerning the ALARM-clause here: http://techcommunity.softwareag.com/ecosystem/documentation/natural/nat824mf/sm/reinput.htm
RULEVAR and INCDIC both come into play with Predict’s automatic or free rules and are not found in normal Natural syntax, though if you ever displayed a map’s raw source that includes such rules, you would see these words.
Though it doesn’t deal with how RULEVAR and INCDIC are brought in, from your perspective, I would recommend reading about map processing rules here: http://techcommunity.softwareag.com/ecosystem/documentation/natural/nat824mf/edis/map_mf_proc_rules.htm#map_mf_proc_rules
After that, if you want more information about Predict’s capabilities, please discuss with your shop’s DBA.
I assume this question was just for curiosity or because someone had such questions on a test you had to take and isn’t an attempt to solve a specific problem. There’s a world of knowledge to be gained in this field for which the documentation will provide answers as would a formal training class.