to give authoritative, detailed direction: to order, prescribe, or arrange what is to be done
διατάσσω is a strong word for putting things in order by authority. It takes the ordinary verb for appointing and arranging, τάσσω, and adds a prefix that pushes it toward the thorough, point-by-point sorting out of what must be done. So it can mean to give someone binding orders, or to lay out an arrangement, an appointment, or a fixed amount.
The two shades run close together. When Jesus finishes instructing the Twelve, or Claudius orders the Jews out of Rome, the word points at people who must obey. When Paul sets the rest of a matter in order for a meeting, or the law is put in place through angels, it points at an arrangement being fixed. Many verses hold both at once, since an order to people usually settles a plan as well.
Because of that range, English Bibles rotate through "command," "ordain," "appoint," "prescribe," and "set in order" for this one verb. Its close relatives keep a narrower focus: παραγγέλλω is passing along a charge, and κελεύω is a spoken official order, while διατάσσω carries the added weight of a deliberate, detailed arrangement.