“tax＋” is for adding tax to a price [×tax rate] and “tax－” is for without tax [÷tax rate]

 

(Included tax calculation for 100 yen)

  "1" "0" "0" "tax+" key：100×1.08＝108

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(Excluded tax calculation for 100 yen)

  "1" "0" "8" "tax-" key：100×1.08＝108

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You can set the tax rate in menu > settings > tax rate