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The first digit of IP67F 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□7 indicates that the GOT is not subject to harmful effects by temporary water immersion.
“F”, the character at the end of “IP67F”, 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.