Hey Mark,
The F numbers (for SMAW) are generally catagorized in groups that are related to the level of skill required to apply the weld.
The lower the number the easier the electrode is to use. (according to this line of thinking)
As far as AWS D1.1 A welder qualified for an F4 electrode may weld on all lower F#s But a welder qualified on a lower F# only may not use higher F group electrodes until he/she qualifies on that higher group.
Here is a link to some detail information about specific behaviors of electrodes in each F# grouping
http://fnumbers.com/An example of an easy to use SMAW electrode would be an F1- E7024. It is designed only for flat welds and horizontal fillets... Can't be used in vert or overhead positions. It has a thick flux coating and is easy to manipulate.
An F2 E7014 electrode has no special handeling (rod oven) considerations and is also very easy to use... While it is rated an all position electrode it works best in flat and horizontal positions and vert down for thin sheet.
An F3 E6010 has no special handling like the 7014 but takes greater skill.. Especially when it is used for open root welds (which is one of it's primary functions) The pipeliners that do amazing open root work (both up and downhill) would have a good argument saying this electrode takes a very high skill level to completely master.
An F4 E7018 requuires special rod oven procedures, may be welded in all positions readily and takes a higher level of skill to master than the F1 and F2 electrodes.