What's the difference between a +IF statement and IF?

I'm guessing probably absolutely nothing at all!

I have seen people using a "+" instead of "=" (it may be an old LOTUS thing), but usually don't see them do both in the same statement. Both the "+" and the "=" usually indicate to Excel that what follows is a formula and not a text string, and they should have the same effect.

The leading "+" is for compatibility with Lotus, which does nothing good or bad to Excel formulas, except that it looks odd. Lotus formulas began with "+", "-", or "@". If the formula has a "-" sign in front of it then that would be significant, but a "+" sign for Excel means nothing significant, in your case sort of like saying =0+IF(H6="START",IU3,""). As you can see, since the leading "+" does nothing and means nothing, it can be deleted.

I figured that since it didn't error out, it had some sort of significance. Makes sense to me though.

As Tom and jmiskey have already pointed out -- it has to do with compatibility with Lotus -- and for this

EXCEL allows you to start a formula with + or - or @ ... and EXCEL will automatically provide the leading = for + and - ; and as a substitution for @