A splinter on a real day
Day 9 at Rosie's Diner has 4 3/8 pages of dialogue. The pickup list also has the insert of the note sliding across the counter (scene 14) and a close shot of the neon OPEN sign flicking off (scene 22). Neither needs the director's full attention or principal cast. The 1st AD adds a line to the call sheet:
SPLINTER UNIT, 2:00P to 4:00P, diner kitchen and exterior sign. B cam operator (Luis), 2nd AC B (Dana), set PA (Chris). Sc. 14 insert (hand double: Maya's SI), Sc. 22 neon sign. Director to approve on monitor.
Main unit keeps shooting in the dining room on A camera. The splinter works in the kitchen with a spare light and a sandbag. At 4:00P Luis and Dana return to B camera for main unit's two-camera coverage.
Why it saves time
Those two inserts would each take a setup's worth of main unit time: maybe 20 to 40 minutes each with the whole crew waiting. Splintering them costs two hours of B camera's availability but nothing from main unit's shot list, as long as main unit doesn't need two cameras during that window. The AD plans the splinter for when main unit is on single-camera setups.
What can go to a splinter
Good fits are inserts, cutaways, signs, screens, and props on a table. Also establishing shots near the location and simple car-bys. Poor fits are anything with dialogue, cast beyond a double, or lighting that has to match a main unit setup exactly. Those belong on main unit or a proper second unit.
Mistakes
Taking crew main unit needs is the common error. If the splinter takes the only 2nd AC, main unit can't reload or slate. Check the crew list, and don't splinter people whose absence stops the main set.
A second error is not telling sound and the script supervisor. Even an insert needs a slate or tail slate and a note in the script supervisor's report, or the editor won't find it. Add splinter shots to the pickup shot list with status and camera roll so they're traceable, and flag them on the call sheet template under a separate heading.
For a list big enough to need a separate location, a company move is the wrong answer; that's usually a second unit day.