The PS3205 will work in one of two modes, block or streaming.
In streaming mode, data is streamed into the PC continuously. This can be done at a maximum sample rate of 1kSps (samples per second).
In block mode, data is collected at 100MSps and stored into RAM on the PS3205 itself. After collection is complete, data can be sent back to the PC across the USB bus at the slower rate.
The maximum sample rate, 100MSps, is only achievable in block mode.