The first digit of IP66F indicates the level of protection against dust (ingress of solid objects).
IP6□ indicates the dustproof type (a wire of 1.0 mm in diameter does not enter).
IP□6 indicates that the GOT is not subject to harmful effects by powerful water jets from any direction.
"F", the character at the end of "IP66F", is a symbol indicating oil resistance.
It is described in the appendix of the Japanese Industrial Standards (JIS) C 0920.
For the details, refer to JIS C 0920.