Hello osi;

To understand this issue better, you must first study the memory structure of the S7 PLCs.
Basically, there are 2 memory areas used by a programmer: Load memory and Work memory.
-load memory is where you download the program and configuration blocks for your program. it is the only area that the user can access.
-work memory is where the program, data and communications are actually processed. The CPU transfers the required data from the load to the work memoryonly whenneeded, and releases it when nor in use; a very efficient way to run a program, and reduces the amount of memory chips required by a Siemens PLC compared to other manufacturers.
One advantage for the user of having distint memory sections in the CPU is that is allows downloads (to the load memory) without affecting the running program (in the work memory) , without having to stop the processor.
Work memory is (except in the case of the largest S7-400H CPUs) not accessible to the programmer; so the only area in discussion now is theload memory.
There is a small amount of onboard load memory in RAM (256kB, generally) on the S7-400 CPUs, large enough for only very small programs. You must add an external memory card to run a decent program, either RAM or FEPROM. Here is how a Siemens FAQ distinguished the two options from the programmer's point of view:

From the FAQ; “S7-400 memory concept”

If you have a flash memory card plugged in, transfer blocks to the external load memory with "PLC >Load user program onto memory card". However, in the case of "PLC >Load user program onto memory card", the CPU has to be set to STOP mode first before the user program can be transferred.
If you have a RAM memory card inserted, the program can be transferred to the CPU via "PLC > Load". The main memory is also updated here at the same time, whereby the runtime-relevant code and data blocks are transferred to the main memory. The internal RAM load memory is filled first. The other blocks are only written to the RAM memory card when it is full.

http://support.automation.siemens.com/WW/view/en/7302549