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Could someone explain the difference between LMT + MKT combo orders and REL + MKT combo orders? I understand the MKT part--as soon as the first leg fills, the other legs are filled immediately so you're (almost) never half in or half out.

However, I don't understand the difference between the first bit: between LMT and REL.

1. In a REL +MKT combo order, are *all* legs submitted relative to the their own leg's bid or ask? Or is it relative to a theoretical spread bid/ask? Why do I have to enter a spread limit price in TWS then for REL, then?

2. For a LMT+MKT, if I set a spread limit of 100.0, does that mean the buy leg's ask minus the sell leg's bid has to come strictly below 100.0 for me to get filled?

Here's a good explanation of REL + MKT:

To use their example, Stock ABC - Stock XYZ is the spread and you want to buy it. ABC is 9.00 x 9.02? and XYZ is 5.01 x 5.02. That means the spread is 3.98 x 4.01. Suppose you send a LMT + MKT with a limit of 4.00. My guess is IBKR will wait around for the ask of the spread to drop below 4 and do it's thing. I'm just not really clear on what that is.

For instance, the order isn't immediately marketable, but let's say ABC and XYZ move to 9.02x 9.03 and 5.00x 5.01, then the spread is 4.01 x 4.03. What I imagine is that IBKR sends out its limit orders at 9.02 and 5.01. Either one leg fills and they do market on the other, or both legs fill and they make even more profit.

But how much does the spread bid have to drop below my limit for me to get a fill? Which strategy is more aggressive?