Recently, I came across this function TO_TIMESTAMP('20-JUL-16 02.25.43.024273000 AM','DD-MON-RR HH.MI.SSXFF AM'). I could not find any reference regarding what X means.
What is the meaning of X in the timestamp format DD-MON-RR HH.MI.SSXFF AM?
That's local radix character. See documentation for Format models.
What's that?
In mathematics and computing, a radix point (or radix character) is the symbol used in numerical representations to separate the integer part of a number (to the left of the radix point) from its fractional part (to the right of the radix point).