Photon Dose Equivalent Hx
Photon dose equivalent Hx (measured in Sv) is a quantity introduced in Germany in 1980. It became the legal quantity in Germany on 01 January 1986. Hx was an interim solution because at that time no international agreement on dose equivalent quantities had been achieved. In Germany Hx was replaced by SI quantities such as H*(10) on 01 August 2001. Hx was not accepted internationally.
Most instruments available up to the early 1980s were designed for Exposure (rate) and calibrated in R(/h). The question arose whether they could still be used to measure dose equivalent. The answer is yes, because Exposure is a good estimate for the dose equivalent of photons in tissue. Therefore Hx was defined as
Hx [Sv] = 0.01 Sv/R x Exposure [R].
Since this conversion does not depend on photon energy, Hx and Exposure are strongly related quantities; they just differ by the factor 100. This is why some allow the user to select either R or Sv as the unit. Basically Hx was not really a new quantity. It was more the old quantity Exposure in a new wrapping.
Why the denomination Hx? Probably because the letter H was already internationally designated for the Dose Equivalent quantities, however there was not yet agreement which suffixes or indexes to append to that letter (H*(10), Hp(10), ...). Consequently the magic »unknown x« had to serve as the index.
Source: Radiation Quantities and Units