I've seen a bunch of questions on this, and I know how easy it is for information to get lost in webnovels, especially when you're reading multiple things, so I'm going to compile information that's directly taken from the book about Runes, Imbuements, and combinations/soul growth. Hopefully this helps clarify a lot of things! I will say that this information is *only* taken directly from the book, so it's not theorycrafting or anything, it's just quotes straight out of older chapters.
“As much as you want, I guess,” Lee replied. “But only an idiot would Imbue too much of their Rune at once. Think of your Rune like a lake. By making an Imbuement, you dig a riverbed connected to it that draws energy out of the main Rune. If you fully Imbue your rune, you’ve just taken the riverbed you just built, filled it with water, and moved it away from the lake. Even though there’s energy in the riverbed, you can only control energy connected to your lake, so you can’t draw energy from it anymore. That’s not even mentioning the soul damage you’ll probably take if you completely split a Rune you formed away from yourself. Why do you ask?”
TL;DR: Imbuements are connections that draw power from the main Rune. If you fully Imbue your Rune, you lose the ability to directly control it as you no longer have any way to use the energy, but this also lets you store entire Runes. Great for Noah/for passing runes from person to person, but there is a chance of taking soul damage when you split away an entire Rune.
I think those are the two most popular questions that should hopefully be answered with lines straight from the text.