Mohammad Rizwan, who is not a part of Pakistan's playing XI for the ongoing first Test against New Zealand in Karachi, led the team out on to the field at the start of the third day, with news coming out that Babar Azam, the regular captain, and two other players were down with "a viral flu".
As such, the laws of the game bar substitutes from leading a side. Rizwan was spotted moving his fielders around but, in theory, any fielder can do that. Word from the team management initially was that Rizwan - the side's designated vice-captain - was the acting captain in Babar's absence, but for reviews, Sarfaraz Ahmed, the former captain who is in the XI after a long gap, was in charge. That came into play in the 53rd over, when Sarfaraz reviewed the umpire's not-out decision when Nauman Ali struck Devon Conway on the pads, and got it overturned in Pakistan's favour.
By then, however, the Pakistan team management had been told that substitutes were not allowed lead a side on the field, and clarified that Sarfaraz, and not Rizwan, was the stand-in captain.
The law in question - 24.1.2 - states: "A substitute shall not bowl or act as captain but may act as wicketkeeper only with the consent of the umpires."
Shan Masood and Salman Agha were the other players who didn't come out to field at the start of play, with the team management informing us about the flu, and adding that they would come out later in the day. Masood did, in fact, come out to field a short while after play started, and Babar and Salman came out in the second session.