This page is reached from Services → IMAP4 Server, then the Advanced tab.

Here are some advanced options for the IMAP4 service in VPOP3

In the IMAP4 service you may have folders which do not exist. For instance, if you are accessing a shared Inbox folder from a user called 'Karen', then their folder will be called #users/Karen/Inbox. The #users and #users/Karen folders are not actually present, but will be displayed in a folder tree. This can upset some older email clients, so VPOP3 can pretend those folders exist, but don't contain any messages.

For strict IMAP4 standard compliance this option should always be checked, but in some cases people have preferred for it not to be checked.

The IMAP IDLE command is a way of implementing 'push' notifications to an IMAP4 client. Normally, you would want this option to be turned on, but in some cases it has caused reliability issues, so it can be turned off if necessary.

This option limits the number of concurrent logins for a particular user. The main purpose for this option is that some email clients will connect to an account and instantly open up a connection for each mail folder which exists in that account. In some cases this can mean that VPOP3 suddenly has to handle 200+ new connections. It will do this, but it may take some time - the problem is that some clients will time out while this is happening, and will reconnect with a new 200+ connections. After a short time VPOP3 will grind to a halt processing new logins for a single user. Most well-behaved clients will limit themselves to 3-5 connections, so allowing 20 concurrent connections will allow about 4 people/devices to access the same mailbox at once. If you know that your clients are well-behaved you can increase this as necessary.

This is similar to the above setting, but restricts the number of logins from a single IP address. This is usually set much higher than the previous setting, because people often connect to VPOP3 over a shared connection, so many users will have the same IP address.