What is the purpose of these system tables(CATALOG, STATS, and SEQUENCE)?

[chenwenhui]

There exists three system tables, named SYSTEM.CATALOG, SYSTEM.STATS, 
SYSTEM.SEQUENCE respectively, in Phoenix4.2.1, but no find any documents to 
introduce these tables' purpose.   if dropped anyone once, what side-effect 
will it produce? and how to recover it?
BTW, it looks like that the SEQUENCE function is no effect after dropped the 
SYSTEM.SEQUENCE, if my applications surely don't use  SEQUENCE function 
forever, whether or not i can drop the table?