Bitcoin Core contains confusing text

I'm trying to locate the following Bitcoin Core texts in the UI, but I do not understand them:

Can not upgrade a non-HD split wallet without an upgrade to support the pre-split pre-keyboard. Please use -upgradewallet = 169900 or -upgradewallet without a specified version.

What's a "non-HD split portfolio"? What is "Pre Split Keyypool"?

Warning: Private keys detected in wallet {% s} with private keys disabled

What is a "private key disabled"?