X-Margin or Cross Margin means that cleared OTC and ETD positions are margined together in a single Initial Margin calculation which takes into account the hedges between the two parts of the portfolio.
The most common Cross Margin offerings are provided by the big Interest Rate Swaps CCPs; CME, Eurex, LCH, JSCC. Fixed Income futures and options to be included in the standard swap margin algorithm are determined based on the benefit of offsets with the swaps compared with the impact of moving them from a 2 day Holding Period to 5 days to be in line with the OTC trades.