That's correct, tony! Those speaker jacks on cabs are connected to each other and act as both input (from the amp) and output (to another cab)!

Most pairs, or trios, of speaker jacks are
connected in parallel, and often say "Parallel Iputs" near them. And two 8 ohm cabs, connected in parallel this way, present a 4 ohm load to the amp. And, as luck would have it, many amp heads often have two speaker outputs and say "4 Ohm Minimum" near them (often with a wattage rating that the cab(s) should be able to handle). While many combo amps have only a single speaker output that says "8 Ohm Minimum", with a wattage rating, because the internal speaker(s) already present an 8 ohm load that will contribute to a total 4 ohm load when another 8 ohm cab is connected.
Parallel connections reduce impedance which then draws
more power from the amp. Like the Rumble 500 is just 350 watts with only its internal 8 ohm speakers, but is 500 watts with an additional 8 ohm cab (giving total 4 ohm load). More speakers and more power is what we usually want.
Series speaker connections, where the ohms simply add to each other (8+8=16), are less common. But they sometimes have a place where you want to add more speakers, but this usually comes at the cost of
reducing the power drawn from the amp! If an amp produces 350 watts with a single 8 ohm speaker load, it will probably produce only 275-ish watts with a 16 ohm series load of cabs. More speakers but less power s not usually what we want.
I hope this helps.
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