These are value-based sources for an item, and will return a positive integer, or a decimal number such as 100, or 0.3. Value Sources will return a positive integer or a decimal number, and can also be split between external and internal sources. External sources are provided by and/or calculated by TradeSkillMaster, such as DBRegionSaleRate from AuctionDB or the RequiredLevel of an item, and internal sources are derived or calculated from other activity you have recorded in the addon, such as NumExpires or NumInventory. There are multiple Value Sources which you can use in your Custom Strings, the following are some examples.

Check the Which Value Sources can I use and what do they mean? page for a complete list, and details on each specific Value Source.