Difference between Portal and Web application access in APM

I referred few articles where it is mentioned that Portal access is for accessing internal webservers with rewrites, if needed whereas Application access is for accessing apps such as RDP. Pls correct me, if I am wrong.

I worked on Bluecoat Reverse Proxy where we have to write rewrite policy explicitly for each and every URL. In f5, I referred some links where they mentioned that using default rewrite profile and creating portal access policy ( by referring only external host name )is enough to achieve rewrite.

Is my understanding correct ? or explicitly we have to mention both external URL and Internal URL ? . If so, where we can point external URL to internal URL. Thank you