When you say user a will check some boxes and user B will check others. Are they working on the same record in the table?
If that is the case, access record blocking will prevent that from happening. On the other hand, they are working on two different records in the table then there is no conflict

The table is a set of records, and each record contains a boolean and an item name. This is displayed in a continuous form (subform). So, if multiple users use the subform bound to that table and click the checkboxes as they work down the list, I believe it will be difficult for them to update it independently. I hope I am describing this clearly.

Record Locks
Watch this video.
You can alter the locking behavior of Access. I have not used SQL server, so I can't tell you if that behavior changes.

Interesting idea, Sami. I noticed, however, that there is no Record Locks property for subforms. I'd have to open it in design mode as a form and then set the Record Locks to the desired level. (I'd want all the records in the table locked.) I'll noodle around with that and see if it works for subforms, as Richard's video describes for forms. Thanks for your suggestion; I appreciate the input.

Go to the Form properties of the Source Object, not the Subform object.

if you have a form with a subform in it, should I only put the main form on record locks or also the subform

It's best to avoid having multiple users editing the same record if you don't have any "team features" to keep things in order, such as not keeping users be kept in the dark from one another.  For instance, in a traditionally-designed forum like this, you have no idea if the user you're replying to is already writing a reply back to you (cross-posting).  But in Discord, you can see the notification that a user "is typing."  This forum shows the "online" yellow indicator next to a user name if the user is online, but only at the moment you load the page.  In more modern forums, user presence is updated in real time.  If the number of users is low (say, 2 to 5), I would probably just do it the simple way and just lock the record whenever a user is on it.  These kinds of measures would only be worthwhile if the user number were high or massive because they could be daunting to design, even on platforms much more advanced than Access.  

This could also be handled in the business side of things instead of the technical side.  For instance, you may avoid delegating the same tasks to multiple users.  If each user does his or her own task, they most likely won't edit at the same records -- and you won't have to design a "multi-user app" per se.  This was how my company did it in my old job: admin users may edit certain tables, but not other users; only accounting users may edit accounting tables, and so on.  I don't think any company would want every user to be able to edit everything.

ok, thanks, I just want to enter new data on the form with 2 people with a split of database and record Locks. Now my previous question, should I also set the subform to record locks if the main form is already set to record locks.

You may have to lock everything the user can see on the screen, because everything on the form (main and subform) is related to one another.  You can't have one user changing the customer from A to B on the main form while another user edits the subform thinking he is looking at the detail for customer A.

Has anyone created a reasonably simple way to prevent multiple people from opening the same form simultaneously in a multi-user setting? The people would all have permission to use the form, but I wish to have only one person at a time use it.

You could just create a system settings table with a value in it like "SharedFormIsOpen" and set it to TRUE whenever that form is opened, and FALSE when it's closed.

System Defaults
System Settings Code

You could add a Yes/No field to the table the Form is based on. When a user opens the Form set the value to True. When they close the Form set it back to False. May run into problems is they try to open the Form at the same time though (the chances are probably less than 1%).

You could even do it with a little text file in the database folder. LOCKED.TXT. As soon as someone opens the form, create that lock file. Delete it when closed.