What is Multi-connection IKM ? And When its required?

What is Multi-connection IKM ? And When its required?

In interfaces if you are using staging area different from target like other databases or internal memory engine that time we need Multi -connection IKM, Oracle providing some of multi connection IKM's like 

1) IKM sql to sql control append

2) IKM sql to sql incremental update

3) IKM sql to file append .....

if we are not selecting multi connection ikm  we need to select Two LKM's

1) one LKM for source to staging 

2) 2nd LKM for staging to target staging (again it will create staging in target)