What's reasons ought to be for moving functionality into subdomain, example,

cusomer portal - clients.acmecompany.com

forum - forum.acmecompany.com

Rather than which makes it

acmecompany.com/Clients

acmecompany.com/help

One valid reason for implementing sub-domain names could be if applications are located on different computer systems. You'll be able to let DNS perform the appropriate routing for you personally whereas using the acmecompany.com/$APPNAME version you would need to setup a front-end proxy. If all your applications are located on a single machine (and will also be for that expected future) this can be a non-problem.

From the user-experience perspective I'd also reason that when the applications look considerably different or require different logins then they must be in subdomains. Basically click a hyperlink and finish on a webpage that appears just like a different application or takes a new login i quickly expect so that it is in a different FQDN. But this time is extremely subjective.

Apart from that I believe it is simply whatever is simpler that you should setup.