为什么在这份英文合同中有两个待签名的地方?

为什么在这份英文合同中有两个待签名的地方?
如下图所示,两个By分别要写什么?（不是一般只写一个人的名字就好了吗?）on后又写什么?之前有看到说写代表谁,可是前面已经有on behalf of XXX了,难不成是写日期?因为它在签名部分的开头写了这样一句话：
“IN WITNESS WHEREOF the Parties have caused this Agreement to be executed as of the Effective Date as follows”:
SIGNED for and on behalf of XXA
by __________________ by __________________
on ____________________ on ____________________
SIGNED for and on behalf of XXB
by ____________________by __________________
on ____________________ on ____________________