In addition to DdlV's info:
(my understanding is that:)
Assets.dta is updated (possibly in realtime) when in Trade mode.
AssetsFix.dta is the basis for the scrollbox, I believe.
My process is to get a (current and relative) update on Assets.dta from the broker (by using the Download.c script), then copy that into a new file named something like this:
Assets-IBFX-140620.dta
which can then be referenced in my script for more realistic simulations.