INFJ and ISFJ are both quiet, caring, and organized, and "Advocate vs Defender" is a common search for a reason: from the outside they behave almost identically. The letter that differs is N versus S, and it shows in what you notice first: the pattern, or the person in front of you.
ISFJ takes care of the people in the room; INFJ takes care of where the room is heading.
The only letter that differs is S versus N.
This pair means strong I, F, and J with an S/N score near the middle. Hands-on responsibility, caring for a household or running a process, pulls people toward S; reflective time pulls toward N. Someone who tests ISFJ during a demanding stretch at home and INFJ during a quiet one is describing the same care in two modes.
Advocate is the nickname some tests use for INFJ and Defender for ISFJ. They differ on one letter, S versus N.
Yes, when your S/N score is near the middle. The two share warmth, privacy, and structure; they differ in whether attention goes to the concrete or to the pattern.
Both read people well. ISFJ reads what someone needs right now; INFJ reads what someone is becoming. Ask which of those you do first.
The four-scenario test scores each area of your life on its own, so instead of one label you see where INFJ shows up, where ISFJ does, and which letter is doing the moving.