What is the difference between moisture regain and water holding capacity?
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".