What is an inventory transaction type?

There are many ways to create inventory transactions. Issuing to shipping, receiving, adjusting inventory, and counting inventory are some examples. Each of these transactions is associated with a unique code (e.g., SH, RP, AD, CC) that identifies the type. These are displayed in many areas throughout the application, mainly within the Inventory module and its reporting screens. You can read more in Appendix A of the xTuple ERP Reference Guide.