You are looking at documentation for an older release. Not what you want? Use the version selector on the top banner or click here.
tosca_definitions_version is a top level property of the blueprint which is used to specify the DSL version used.
For Cloudify, the versions that are currently defined are cloudify_dsl_1_0, cloudify_dsl_1_1, cloudify_dsl_1_2 and cloudify_dsl_1_3.