I believe it is the % of units sold of a product from total qty of all products sold.
Similar to the % value for product group on work period but by qty rather than value.
Ie 100 x £1 coffees would give higher % than 4 x £25 steaks.
In an example if sales for a period were;
100 x Coffee £1 = Total Product Sales @ £100
4x Steak £25 = Total Product Sales @ £100
50 x Beer £4 = Total Product Sales @ £200
The sales % as on work period would show;
Coffee = £100 = 25%
Steak = £100 = 25%
Beer = £200 = 50%
Total Sales = £400 (100%)
The participation sales would be based on QTY giving these results;
Coffee = x100 = 64.9%
Steak = x4 = 2.6%
Beer = x50 = 32.5%
Total QTY= x154 (100%)