Typical order is well pump → pressure tank → sediment/iron/softening treatment → UV as appropriate → house. Follow the specific equipment manual when air-injection, chemical feed or specialty tanks change the design.
Why treatment usually follows the pressure tank
The pressure tank stabilizes system pressure and limits pump cycling before water enters treatment. Pentair explicitly instructs well-water installations to place its filtration after the pressure tank.
Do not put a restrictive filter between pump and tank casually
The pump, pressure switch and pressure tank are designed as one pressure-control system. Adding unapproved restriction in that section can create operating problems.
Special treatment can alter the layout
Chemical feed, retention tanks, micronizers and certain aeration systems can have different placement requirements. Use the manufacturer schematic for those designs.
Design the complete train before cutting pipe
Plan bypass valves, drains, electrical outlets, UV placement and service access before installation.