Water retention is how much water the fibre can retain. Water holding is function of hydrophilicity and construction of the fibre (fabric). What is the difference between those two? Because to retain is also to hold, in my knowledge.
According to the FabricLink Textile Dictionary:
Moisture Regain - The amount of water a completely dry fiber will absorb from the air at a standard condition of 70 degrees F and a relative humidity of 65%. Expressed as a % of the dry fiber weight.
Water Holding Capacity is the amount of water a completely dry fiber will retain after soaking in liquid water and allowed to "drip dry".