F5 LTM (ADC) question

Hi AhmedMS , 
Virtual server port : is the port that you assign to the virtual server to listen on , for EX: you need to publish a { http } Application , so you shoud create a virtual server ip with the proper service port { 80 for http } to map all connections which destinated to your http application via this virtual server. 
NOTE : you do not have to assign port 80 for http traffic you can use any custom port , I use it for simplicity. 

The Alias service port in Monitors : from its name it is a monitoring related , you want to create a custom monitor to check the availability if this application runs this port properly or not. 
For Example : you have a application server listens on port 777 and you need to check if this service port up and available or not in this case , you do not only assign a normal tcp monitor which perform only {SYN , SYN-ACK , ACK} you need to be more specific and check the service itself up and running or not , so when you create an Alias with service port "777" it will go to check the service itself periodically it will ask for "777" I want to say that.

this is widely used when you want to monitor the Backend servers which listen on differnet ports other than the pool members service ports. 

I hope you find this answer insightful.