The default behaviour seems to be for the value of a control (e.g., a knob or text field) to immediately be reflected in the rest of the software. However, I want to have a section of controls on my VI whose values are only propagated when an "Apply" button is pressed. This allows the user to ensure that multiple controls can have their values changed at the same time which might be important (in my case, I aim to make LabVIEW control DC power supplies).
You need to understand data flow. The new value of a control is not read by the code until data flow allows. We need to see the rest of the code to give specific advice, but one possibility would be to place all terminals into an event case that only fires on a value change of the "apply" button.