To understand subform list questions, you must understand the use of subforms in Magpi.
To illustrate subform lists, imagine you have a form and two subforms:
1 form "Household"
1 subform "Kids" for all the children in the household (to collect age, gender, vaccination status, etc)
1 subform "Nets" for all the bednets in the household (to collect brand, age, etc)
The form Household will have two subform questions to allow the data collector to "jump into" the subform "Kids" and repeat the Kids questions until all the children in the household have been discussed, and similarly to allow the data collectors to jump into the subform "Nets" and enter all the net information.
Subform list is a type of question inserted into Nets or Kids (not into Household). For example, if you first entered data into Kids, and then begin entering data into Nets, you can insert a subform list question into Nets that allows the data collector to view, and to select, which child sleeps under that net. Or, if you entered Nets first, and are now entering Kids, you could insert a subform list question into Kids that displays a list of all the entered Nets, and allows the data collector to select which bednet each child sleeps under.