These three reports are designed to look for errors in the way products are set up that will effect/prevent their ordering via the Suppliers|Orders|Generate Orders facility.  The following are extracts from their explanatory text:

Product Check - Quantity Setup:

This report lists products where there are errors in the ideal and critical quantity settings.
Errors are:
 - if the critical quantity is greater than the ideal quantity
 - if either of the ideal and critical quantities is less than zero
 - if the critical quantity is zero
 - if the ideal quantity is zero
Note that the last two are not really errors - but they will prevent the product being ordered by the Suppliers|Orders|Generate Orders function.
Inactive products are always ignored. Inactive and non-current suppliers are ignored.
Products with critical quantity set to zero will not be show unless the 'Include Critical Qty=0' box is checked.
Products with ideal quantity set to zero will not be show unless the 'Include Ideal Qty=0' box is checked.
Unless the 'Include Zero Use Products' option is set, products with zero usage in the specified period are ignored.

Usage is calculated for each selected stock location for the last N weeks where N is set by the 'Weeks for usage calc' parameter. (If the date is say 20/9/2014, then setting a 6 week usage period will calculate usage from 9/8/2014.)
'Usage' means sales less any credits, and does not include any stock transfers.

If you check the 'Check critical & ideal against usage' option then the ideal and critical quantity settings are checked against the usage. This is done by comparing the critical and ideal levels against the equivalent weeks usage. For example if the usage is 36 for 12 weeks, and the Ideal Weeks Usage is set to 3, then an error will be flagged if the ideal quantity is less than 3x(36/12)=9. Similarly if the Critical Weeks Usage is set to 1 then an error will be flagged if the critical quantity is less than 1x(36/12)=3.  These numbers are rounded to the nearest integer - ie 2.75 becomes 3 and 0.25 becomes 0. The suggested values are shown in brackets in the (sug) columns are are only shown if the suggested value is larger that the currently set value.

Product Check - Supplier Setup:

This report lists products where there are errors in the supplier setup for the product. Errors are indicated by a question mark in the left column.
Errors are:
 - if the Automatic Price Update flag is set but the Supplier List Price is zero (AU column will show !!)
 - if the Supplier's Package Size is 0 (Size column will show !! 0)
 - if there is more than one preferred supplier set - preferred suppliers have an asterisk before their name (Pref.Count column will show !! 2 if there are two set)
Note that the 'no preferred supplier' error is checked for by the 'Product Check - No Supplier' report.

Unless the 'Include Zero Use Products' option is set, products with zero usage in the specified period are ignored.
Inactive products are always ignored. Inactive and non-current Suppliers are ignored.

Product Check - No Supplier:

This report lists products which will not have orders generated by the Suppliers|Orders|Generate Orders facility because they have both no preferred supplier set and no preferred supplier has been set for the stock location.
Unless the 'Include Zero Use Products' option is set, products with zero usage in the specified period are ignored.
Inactive products are always ignored. Inactive and non-current suppliers are ignored.
Usage is calculated for each selected stock location for the last N weeks where N is set by the 'Weeks for usage calc' parameter. (If the date is say 20/9/2014, then setting a 6 week usage period will calculate usage from 9/8/2014.)
'Usage' means sales less any credits, and does not include any stock transfers.
If you set a Stock Location selection, then only these stock locations will be examined to see if they have a preferred supplier set.