Build.scala, % and %% symbols meaning

From the official documentation:

http://www.playframework.com/documentation/2.1.1/SBTDependencies

Getting the right Scala version with %%

If you use groupID %% artifactID % revision instead of groupID % artifactID % revision (the difference is the double %% after the groupID), SBT will add your project’s Scala version to the artifact name. This is just a shortcut.

You could write this without the %%:

val appDependencies = Seq(
  "org.scala-tools" % "scala-stm_2.9.1" % "0.3"
)

Assuming the scalaVersion for your build is 2.9.1, the following is identical:

val appDependencies = Seq(
  "org.scala-tools" %% "scala-stm" % "0.3"
)

As you can see above, if you use %%, you don't have to specify the version.

This is part of SBT which play uses as a build tool. Specifically this is an import statement.

The percent symbol % is a actually a method used to build dependencies. The double percent sign %% injects the current Scala version - this allows you to get the correct library for the version of scala you are running. This is to avoid having to change your build file when you update Scala.

More information here