Apart from Existing answers,you will need to know how RAM is utilised by SQL Server.
Suppose you have a box with 100 GB RAM dedicated for SQL Server only.SQL Server will utilise all this RAM until you constrain it and lets say you constrain SQL Server maximum memory to 94 GB...
Lets understand how this 94GB memory is used and what are the main components that use this memory..
--running below DMV gives me total list of components which may use this 94GB RAM
select (sum(pages_kb))/1024 as 'sizein_mb',type as 'clerkttype'
group by type
order by (sum(pages_kb)*128)/1024 desc
As you can see from the above output,There are many components that are using memory right now and also We can see Buffer Pool is the largest consumer of memory ,followed by plan Cache,lock manager..
One important point to note is the above components would adjust memory among themselves.Say for example,Memory utilized by plan cache will be stolen from buffer pool,so if your plan cache is filled up with Adhoc plans,buffer pool will have less memory to operate and it in turn forces pages to disk which is not good..
Now you have understand how the RAM you set as limit is used,its good to know what are the objects that can use how the 6 GB RAM you left to OS is utilized..
Below are the components that derive their memory in 6 GB
3.Extended stored procs
3.third party dlls or addins
6.SSAS and so on
Now that you have understood how SQL utilizes the RAM,you can easily make a decision and start troubleshooting if any issue arises as well