The filling material inside of HPLC columns is called the stationary phase. The common stationary phase for HPLC columns can be silica-based or polymer-based (native or synthetic). Once filled into the column it does not shift. Due to the polarity depending from the modification type they are divided into reversed stationary phases and normal stationary phases. Dependent on your chosen material the stationary phase has different characteristics.
Please follow carefully the KNAUER Column Care and Use recommendations for different column types.