It only appears if one of the variable names has a number at the end and there is a factor involved. In such a case the function adds a suffix ".x" to the variable names in order not to confuse the normal variables (with numbers at the end) with factors (see here)
However the back-transformation of the names of factors is not correct, especially for names of factor varaibles, since their name is e.g. myvar.x.1. See example of Gomaa.
It only appears if one of the variable names has a number at the end and there is a factor involved. In such a case the function adds a suffix ".x" to the variable names in order not to confuse the normal variables (with numbers at the end) with factors (see here)
However the back-transformation of the names of factors is not correct, especially for names of factor varaibles, since their name is e.g. myvar.x.1. See example of Gomaa.