In case you have several vehicles (trains, trucks etc.) waiting for one type of cargo on a station it is much more effective to force loading an arrived vehicle and only after it is full and ready to go start loading next. Note that this is all the patch does; it has no FIFO attributes, and does not depend on which vehicle came first.
This patch is disabled by default. You can enable this setting through the advanced settings -> stations window or use the following syntax in Openttd.cfg:
[settings] improved_load = true