Ryobi CS30 & Homelite ST145 Trimmer Replacement (3 Pack) RH Spool Retainer # 308042002-3pk Genuine OEM Replacement Parts (3 Pack) It has a right hand thread screw to remove and attach it to the trimmer head. Compatible with Ryobi and Homelite Brand Trimmers Its purpose is to hold the spool with the cutting line, in the spool housing.