Importing Historical Data



could you help me to understand how historical data works in MT4? for example, when we import data from other sources (for example yahoo), where this data is stored?, what symbol is the container? i am importing data from a .csv (previusly converted data into mt4 format) but i don't know what's happens if mt4 goes online and recieves data from the broker, and what about .hst files.

Thanks in advance