TLB files are primarily used to hold type information externally to a DLL. Some kinds of COM libraries were not originally meant to hold the sorts of information a VB program would normally use for linkage, and so later TLBs were created to "bolt on" this functionality.

TLBs are also used to define abstract interfaces. The sorts of things you'd use a VB Implements keyword with.

Another use is redirection. In a system with MDAC 2.8 installed there is only one ADO library. What appears as older versions when you go to add a reference are TLBs that provide the old interfaces but point to the sole ADO library on the system.

Thus many people labor under the impression they have several versions of ADO installed, when they actually only have just the one. The extra TLBs allow you to develop against the older interfaces, and by choosing the lowest version your program needs you can ensure portability.

For example by using a reference to ADO 2.5 when you build on an MDAC 2.8 system, your program should work on any system with ADO 2.5, 2.6, 2.7, 2.8, or DAC 6.0 installed. Barring bugs in ADO 2.5 on a system that actually only has 2.5 installed.