it boils down to back when they made these machines. They figured at the time that having too many "watts" of speakers hooked up would cause the amp to blow. (of course we know that to be absolutely incorrect now.)
the idea was that you had these "taps" that dropped down the "watts" being delivered to the speakers to keep the output of "watts" under the maximum output capability of the amp. (in the case of these units, 250 watts.) if you understand the real reason for amp failure you can see how absolutely ridiculous the whole thing is. The had a great big calculator sheet and everything in the manual.
of course now, we know that having an insufficient load on the amplifier is the biggest contributor to amp failure, not too many watts. You must keep the attached load of speakers over 4 ohms per channel. Having too many speakers (improperly wired to have a low load), act as a dead short causing the transistors in the amp to blow.
you can place the speakers anywhere along the strip you like. basically the lower the number the lower amount of sound you will be able to get out of your speakers. The highest tap (E7) is directly attached to the output of the amp. (no portion of the transformers mounted on the plate are used.) The lower numbers (E6-E2 have an ever increasing ammount of transformer turns increasing resistance on the speakers.( basically having a dummy or phantom speaker attached in series)