When any texture layer is created in the Shader Tree, image based or procedural, an associated 'Texture Locator' item is automatically generated and added to the items list and linked by the texture layers 'Locator' property. By default, Texture Locators are generally added to a folder (really a group locator item) called 'Texture Group'. This makes it easy to toggle the visibility of the locators in the 3D viewports, as some users find them distracting. Users may select the texture locator directly in the items list, or by selecting the locator icon in any 3D viewport. Most often though, users will select them via the shader tree; when any texture layer is selected in the tree, the locator is available in the named subtab on the right side of the items associated 'Properties' panel. Additionally, the locator may be toggled by the small '+' sign preceding the texture layers name in the Shader Tree, LMB+clicking the '+' icon reveals the layer.
Texture locators provide a very important function as they control the application of the texture layer over the rendered surface. A Texture Locator is like a regular 'Locator' Item, in that it has Position, Rotation and Scale information. Additionally there are settings specific to controlling the images projection method, meaning how it is mapped or applied to the surface. For certain projection modes, such as 'Planar', users can interactively position textures in the 3D viewport by transforming the Texture Locator item itself. Other modes, UV in particular, completely ignore the positional and scale attributes as the mapping is defined by the UV map itself.
The attributes of the Texture Locator item found in the properties panel are documented on the Texture Locator page of the Shading Lighting and Rendering section of the documentation, please reference that page for additional information.
