The Cost without Discounts measure currently removes automation of various discounts Enterprise Program Discounts, Solution Provider Discounts, and Private Rate Discounts.

When these discounts are automated by AWS a number of Net Fields are added to the CUR - Net Unblended Cost and Net Effective Savings Plan Effective Cost. These apply the discount directly to Line Items associated with Usage rather than representing them as a lump sum.  Cost History will make use of the Net Fields across the Cost Dimension when they are present.

Within the CUR the automated discounts will still display their lump Sum credit values across the Unblended Cost Dimension however if you check Line Item Type via FlexReporting.

Cost Without Discounts pulls the value from the Unblended Cost field so you will find for example EDP Discounts return the full credit value when checking Detailed Charge Type and using the Cost Without Discounts measure, whereas they will either not be present or lower when using the Cost dimension as the credit is being applied directly to Usage.

Cost Without Discounts doesn't remove Discounts associated with Savings Plans and Reserved Instances currently.