Requirements / target filters and subsituable / selectable are too confusing.

So in my opinion we should: Move target_filters to the node type's requirement section as it make sense here: I create a reusable implementation of a MySQL that is based on sh scripts so I want to be able to specify the host requirement to specify that the os type is linux. Remove target_filters from the node templates requirement section. Remove the directives and keep only an abstract keyword that is false by default (or substitutable if abstract is not clear enough). I don't seen the need for directives right now, a single substitutable seems to do the job and is quite simpler in my opinion. Make Compute node abstract (or substituable) - same as block storage or other IaaS components. Note that a type may be abstract too and not just a node template. I don't see the scenario of a Compute node that is not selectable somehow as this is instantiated on the cloud... For me Compute node is by definition an abstract node that is substitutable and the orchestrator should find a valid match to instantiate a VM with the right required contraints or find a real machine if bare-metal is supported by the orchestrator. Use the substitution mechanism for both substitution and selectable in a topology. The orchestrator implementation is responsible for providing the valid mechanisms to put in front. At the end that leaves the TOSCA user with only 2 notions and use cases which is much simpler to explain and understand and leaves also in my opinion more flexibility both to users and implementors. On a type - or substituable type (that defines a reusable entity) I can specify requirements with target_filters to make sure that when someone reuse my entity it will be a compliant usage based on my nodes real requirements. In a topology template every node that the orchestrator must provide (by offering selection or automatic best-match or both – specific to implementation and vendor) must be defined as an abstract (or substitutable) node. PS: I'm really sorry I was not able to make it to the tuesday call as I know I come a bit late with my points but still hope this is makes sense for you too.