Comment 1 for bug 662398

I might not be 100% accurate and everyone is free to correct me... but here's the main idea:

The purpose is to avoid API and ABI breakage between your program and the library if there is a major .

Your package should actually only have build-depends to libldap2-dev in the debian/control file, and the packaging toolchain will automatically add a dependency to the specific binary package (libldap2.4-X) during the build process.

For example:
Build-Depends: debhelper (>= 7), libgstreamer0.10-dev, libxml2-dev, libglib2.0-dev

You might be also interested in http://sourceware.org/autobook/autobook/autobook_91.html (especially the "age")