How SR-DSDS and DR-DSDS work?

Anyone know Concept of SR-DSDS (Single Receive Dual SIM Dual Standby)
DR-DSDS ( Dual Receive Dual SIM Dual Standby )?
How many antenna & How is works ?
Why DR-DSDS only work when One of the sim is register with Vowifi ?

This is concept of Qualcomm anyone help for More Details