Using "what If" form to estimate taxes when I turn 72 / Premium Tax Credit or Repayment Smart worksheet

I wanted to use the "what If" form to help me "predict" my possible tax bill when I turn 72 and have to take a RMD from my 401K

 

My 2019 tax situation is pretty straightforward. I qualify for the standard deduction, my income is SSA , a $500/mo pension, and a $10 K distribution from an inherited IRA, which I closed out in 2019.

 

I used the data from 2019 and copied over the data into a new column.

 

The only value I changed was I  "upped" my IRA distribution fromm the $10K used in 2019 to a value greater than what I expect the RMD to be when I turn 72.

 

That bumped my total income, as expected. It bumped the SSA taxable amount, as expected

 

Another field "popped up" and populated on the Premium Tax Credit or Repayment Smart worksheet  on line C (tax credit).  This worksheet was blank when I filled out my 2019 taxes.

 

Can someone outline to me what the worksheet is used for ?

 

I tried reading the IRS.gov site for information, but I can not cut through the jargon (for me) to figure it out.

 

Thanks!!!!