spread的计算理解

Notes里有这么一段：

The premium trading value of OTR bonds is due both to their liquidity and financing advantage as we previously discussed. The liquidity advantage stems from the ability to sell these bonds quickly for cash. The financing value stems from the ability to lend the bonds at a cheap special rate and use the cash to lend out at higher GC rates. This financing value is dependent on the trader’s expectation of how long the bond will continue trading at its special rate before the rate moves higher toward the GC rate.

Let’s assume that an OTR bond is issued on January 1 and trades at a special spread of 0.18%. 

这里special spread的计算是怎么个过程啊，我理解的是GC-SC，那么GC和SC怎么可以跟这里的OTR bond进行对标呢，烦请指导讲解。