The Strength Factor (SF) is a concept of the USFA, an applies only to US fencers.
This page discusses only the Senior SF system.
The formula for the SF is published in the Athlete's Handbook, which you can find on the USFA website. The idea behind it is, more or less, that an event which is twice as strong as another event will have twice as large a SF. In most circumstances it reasonably approximates the strength of an event. (At very small tournaments it doesn't work so well.)
The SF is used to multiply the points earned at foreign tournaments and all World Cups, so equal places at a strong event and a weak event will get more and less points respectively. The USFA caps the multiplier at 2.0.
The formula is this:
Number of entries divided by ten
plus Number of fencers ranked 1-8 in the world multiplied
by seven
plus Number of fencers ranked 9-16 multiplied by
six
plus Number of fencers ranked 17-32 multiplied by
five
plus Number of fencers ranked 33-64 multiplied by
four
plus Number of fencers ranked 1-16 in Junior World standings
multiplied by three
plus Number of fencers ranked 65-100 multiplied by
two
That whole sum is divided by 100.
One fencer can only occupy one slot, so a Junior 16 who is in the top
64 only counts in the top 64 slot.
The formula is conveniently abbreviated like this: (N/10+top8*7+top16*6+top32*5+top64*4+j16*3+top100*2)/100.
If every person in the top 100 shows up at an event, plus 100 more fencers (and no j16), what will the strength factor be? N=200, top8=8, top16=8, top32=16, top64=32, j16=0, top100=36. (200/10+8*7+8*6+16*5+32*4+0*3, 36*2)/100 => 4.04.
Call that scenario "100% attendance". As such, the maximum practical SF is 4.0. In theory it could get to about 4.5, but I don't think that could or will ever actually happen.
The concept of attendance.