When set to In memory: Input need not be sorted, the 
component accepts unsorted input and permits the use of the 
maintain-order parameter. Component sorts records 
internally in its buffer.

When set to Inputs must be sorted, the component requires 
sorted input, and the maintain-order parameter is not 
available.

Default is Inputs must be sorted.

The Inmemory sort and input must be sorted options are 
there in the Join,Rollup and Dedup components.
Main difference between these two is if you selected input 
must be sorted options in the above mentioned components 
the the downstream components will get the records in a 
sorted oder. if you are selected option as Inmemory sort 
then the downstream components will not get the sorted 
records.