You said that you saw this number under "Other Tax Situations". You appear to be using TurboTax Self-employed Online. In this section, I had to guess where you saw this "2017 Tax Liability". I assumed it was under the subheading "Additional Tax Payments" in the section "Underpayment Penalties".
This means that you are dealing with the 2210, the calculation for underpayment of estimated taxes. The reason that information from your 2017 return is being sought is because one of the exceptions to the underpayment of estimated tax penalty is if the amount of tax you withheld and paid during the year was greater than or equal to the tax assessed in 2017. This is why you have those intense instructions, because you asked where this 2017 Tax Liability came from.
Like many things in tax, it is not simple. There is a good reason that TurboTax has fetched this number. Unless you have good reason to think that TurboTax did it wrong, I suggest that - as you said - "let TurboTax do what it does with it & continue on".
