The status bar shows whether the current model has been saved, loaded, merged, changed etc.
Other items appearing on the status bar are:
current entry number, level in the model (+ number of descendants, if any)
entry colour status:
E=Entry has its own colour
M=Model colour is set and the entry is transparent
P=Model colour is not set, the entry is transparent and
is therefore showing according to BrainStorm preference settings.