For an equal N-way RF divider, split loss is the unavoidable 10 log10(N) dB reduction caused by sharing the input power among N outputs; excess insertion loss is the additional loss of the real device beyond that ideal value. A 2-way ideal divider has 3.01 dB split loss, a 4-way has 6.02 dB and an 8-way has 9.03 dB.
If a 2-way divider measures 3.55 dB from common input to one output, the excess loss on that path is approximately 3.55 - 3.01 = 0.54 dB. The 3.55 dB value is the total path loss. A data sheet that lists 0.54 dB may be reporting excess loss only, so the column definition must be confirmed before products are compared or a link budget is calculated.
Excess loss does not replace the other acceptance checks. Measure both output paths under the same terminations and reference planes, then verify amplitude imbalance, phase imbalance, return loss and output isolation. At high power, repeat the check under representative average, peak, duty-cycle and mismatch conditions because heating or termination stress can change the practical result.