ISTJ and ISFJ are the two most dependable types and the two most often mistaken for each other, including by themselves. Both are quiet, practical, and allergic to drama. The letter that differs is T versus F, and it shows when duty and people pull in different directions.
ISTJ keeps the promise because it was made; ISFJ keeps it because of who it was made to.
The only letter that differs is T versus F.
This pair means strong I, S, and J with a T/F score near the middle. Work often tips it toward T, because procedures and results reward directness; home and close relationships tip it toward F. If you test ISTJ from your desk and ISFJ from your kitchen, that is the pattern, not a mistake.
If your T/F score is near the middle, yes. The two share everything except how they decide when duty and people conflict, and many people decide differently at work than at home.
Yes. Logistician is the nickname used for ISTJ and Defender for ISFJ.
No. ISTJ can be deeply kind and ISFJ can be firm; the difference is what settles a decision when being fair and being kind point different ways.
The four-scenario test scores each area of your life on its own, so instead of one label you see where ISTJ shows up, where ISFJ does, and which letter is doing the moving.