The FPSS boards add up the cluster counts and pass this information along to the FPTT under normal operating conditions. When a L1accept occurs the upstream DFEF boards momentarily suspend L1 record transmission and transmit a large L2 record containing a sorted list of clusters. The FPSS boards combine these cluster lists, sorting and truncating as needed, before sending the data onto the L2 processor farms. The FPSS also sends it's L1 inputs to the L3 readout crate.
The FPSS Technical Design Report is aviailable as DZERO note 4789.