we have to use process the only one application  at time and
update the database is asynchronously and synchronously.then
we have to use call transaction in BDC

IF we  are updating data by Online updation mode then we 
will go for call transaction method. And in case of 
Assynchronous updation then also we will go for Call 
transaction. Bcoz we can't run session in Background

Call Transaction:-
When there is need of only one transaction to be updated 
such as 'MM01' and number of records are less. And also 
while updating the database there will be less load on the 
processor, as we know call transaction works in foreground 
i.e in online mode then we can use call transaction.

Otherwise use Batch input session as it can update mor than 
one transaction and it is used when the records are more as 
it can be processed in background processing mode. Its 
better to use batch input session as it provides error 
handling facility implicitly whereas in call transaction we 
have to write code i.e explicit error handling.

thanks & regards

i would like to add one more point.

In order to Handle dependent Transactions, we should use
Call Transaction Method. Because we can't handle dependent
transactions in session method.