Expand on relationship between OSMP and SSP

Signed-off-by: Pierre R. Mai <[email protected]>

Author: pmai

doc/spec/model_types.adoc

@@ -4,7 +4,7 @@ include::{root-path}_config.adoc[]
 endif::[]
 = Model types
 
-The current specification supports packaging the following model types as _Functional Mock-up Units_ (FMUs). A model can alternatively be packaged as an OSI-compliant system according to the _System Structure and Parameterization_ (SSP) standard, consisting of multiple FMUs.
+The current specification supports packaging the following model types as _Functional Mock-up Units_ (FMUs).
 
 Environmental effect model::
 This model type can be used to model environmental effects or the physical parts of sensors.
@@ -35,3 +35,5 @@ All models may also consume a global `osi3::GroundTruth` parameter during initia
 
 Complex models may combine various aspects of the above model types.
 Manual intervention is needed to configure and set up these FMUs.
+
+In implementations that support the use of the _System Structure and Parameterization_ (SSP) standard, a model can alternatively be packaged as a system consisting of multiple FMUs, if it presents the same interface at system level as this specification gives for the overall model of the given type.