Every remaining credit is either nonrefundable or refundable. Nonrefundable credits — foreign tax, care expenses, education, saver's, energy, general business, elderly or disabled, mortgage interest, clean vehicle — reduce tax but stop at zero; they live on Schedule 3, Part I (with the child credits on the main form). Refundable credits and payments — net premium tax credit, EIC, additional child tax credit, American opportunity refundable portion, refundable adoption portion, withholding, estimated payments, excess Social Security tax — can reach past zero into refund; they live in Schedule 3, Part II and the main-form payment lines.
Order matters within Part I: each credit-limit worksheet subtracts the credits already claimed, so the foreign tax credit applies before the care credit, the education credits yield to earlier lines, and the adoption credit's worksheet accounts for lines 1 through 4 and the later-line credits ahead of it. The retirement savings contributions credit (saver's credit) rewards IRA, 401(k)/403(b), SIMPLE, SEP, and ABLE contributions, but bars anyone born after January 1, 2008, anyone claimed as a dependent, and any student, and phases out above the listed AGI thresholds. The home energy credits — residential clean energy and energy efficient home improvement — run through Form 5695 against the residence and main-home conditions.
Worked example
A single filer with $4,000 of tax claims a $900 foreign tax credit and a $1,100 lifetime learning credit. The foreign credit applies first, leaving $3,100 of tax; the education credit then applies against the remainder, leaving $2,000 of tax. Had a $500 saver's credit also been in play, it would come after both — and any credit the $2,000 remainder cannot absorb stays unusable, because no nonrefundable credit can reduce tax below zero.