What the different between SRCROOT and PROJECT_DIR?

Exchangeable in practice, while the documentation makes these subtle theoretical distinctions:

1. SRCROOT

   Directory path. Identifies the directory containing the target’s source files: contains the path to the project file that defines the target.

   SOURCE_ROOT is an undocumented alias to SRCROOT

2. PROJECT_DIR
   Identifies the directory containing the project (.xcodeproj)
   $(PROJECT_DIR)/build is used as the create the default value for:

   • Intermediate Build Files Path OBJROOT
   • Build Products Path SYMROOT

   Typically these paths are not set per target, but are set per-project or per-user.

3. PROJECT_FILE_PATH
   Identifies the project itself.
   Equivalent to $(PROJECT_DIR)/$(PROJECT_NAME).xcodeproj

Conceptually different (#1 is about the project which defines the target while #2 is about the project independently of any target), they are always pointing to the same location since you are, in essence, always building a target.

References

Xcode 8.3 Build Settings reference
Xcode Build System Guide (Retired 2016-09-29)

SRCROOT & PROJECT_DIR are same macros have same effect.PROJECT_DIR makes more sense when talking about frameworks.