There is only one file type that uses the .PBXBTREE extension:
Xcode Auto-Complete File
File created by Xcode, a software development tool for Mac OS X; contains references to functions, protocols, imports, and other data; used for code hinting and autocompletion by Xcode's CodeSense component.
PBXBTREE files save a binary tree ("btree"), which provides fast access to autocomplete information as the developer types characters. The PBXBTREE files are located within a project's build directory. For example, for "MyProject," they are stored within the /build/MyProject.build/Project.pbxindex/ directory.
NOTE: PBXBTREE files are automatically generated by Xcode. Therefore, if you delete them, Xcode will regenerate them.
Updated: October 10, 2010