eZ430-RF2500

I wanted to have ez430-RF2500 chip and CC2544 chip communicating by SPI. I went through slau144i document for ez430-RF2500.

1. In chapter 16, page 451, the master mode of SPI has a UCxSTE connected to SS. Is UCxSTE the same as slave select? Then why slave select is input in master? Can someone briefly explain how does slave select work in SPI communication?

2. When does SS work after writing data to UCxTXBUF?

Thanks