On the maximum deferral of self employment tax payments part 3 line 18, I don't understand

This question is associated with the option to defer paying some of your self-employment tax due to Covid.  

If you do not want to defer any of the self-employment tax, then enter '0' for line 18.  

If you do wish to defer paying the self-employment tax, then line 18 is asking for the portion of your Schedule C and/or Schedule F profit that is attributed to March 27, 2020 through December 31, 2020.  

Here is some information from the TurboTax help content that should give you a better explanation of what you might need to enter:

How do I calculate my eligible income?

The amount of the self-employment tax that is eligible for deferral depends on your net income from 3/21/2020 to 12/31/2020.

If you made roughly the same amount each month, calculating your eligible income is easy. Just grab a calculator and multiply your net self-employment income by 0.775.

If your income was not consistent from month to month, you'll need to do some extra math to find your eligible income. You can still use the amount we gave you for your net self-employment income, but you'll need to subtract the net income your earned between 1/1/2020 and 3/20/2020. Remember, your net income is what you earned minus any expenses.

Here are some examples:

Example 1: John's annual gross income from self-employment work last year was $60,000 and he had $6,000 in expenses.

First, John needs to figure out his net income for the year. He does this by subtracting his total expenses from his gross income ($60,000 - $6,000). This gives him a net income of $54,000.

Next, John needs to figure out his net income from 1/1/2020 through 3/21/2020. His total income during this period was $20,000 and he had expenses of $1,000. He subtracts his expenses from his income ($20,000 - $1,000) to get $19,000 in net income for this period.

Now John can figure out his eligible income. He subtracts his net income from 1/1/2020 through 3/21/2020 from his total net income ($54,000 - $19,000). This gives him eligible income of $35,000, which is the amount he enters as income eligible for tax deferral.

Example 2: Jane is self-employed and has already entered all her income info into TurboTax, which shows her total net self-employed income as $54,000.

Jane had a slow start to the year, and only earned $2,000 between January 1st and March 21st. She also had $2,000 in expenses. She subtracted her expenses from her net income ($2,000 - $2,000), which gives her a net income of $0 for that period.

Janes eligible income for the self-employment tax deferral is $54,000 ($54,000 - $0).