PHOENIX and LySI are so-called "Theoretical" services, or TSA, in the sense that the standard mandatory parameters of a SSA, POS and SIZE, are not relevant. These services are still implemented as SSA.
The FORMAT=METADATA query returns a VOTable describing all the mandatory and optional parameters supported by the services. Our theoretical services support the mandatory parameters POS and SIZE, and the FORMAT=METADATA response describes the behaviour of the service as an INFO tag (which cannot be automatically interpreted by a client). Therefore, the only chance for a client to identify that POS and SIZE are not relevant for a particular service is to describe it as a TSA (rather than SSA) in the registries (this is in fact the only difference between TSA and SSA). If either POS or SIZE are specified by the client, the services will return no matching data.
The format of the mandatory and optional parameters is described in the IVOA document, but there is no guideline concerning the general (non-standard) parameters of a particular service. These non-standard parameters are fundamental for TSA services.
The different implementations of TSA, which actually set the need for TSA and were used as "reference implementations" match the client's provided value with a strict equality, and list the whole choice of acceptable values for a parameter as a series of <OPTION> in a <SELECT>. This mechanism becomes tedious, and possibly non-scalable, when the number of choice is very large. It is even inappropriate for a service that supports input parameter distributed continuously, as for LySI or generally for a computational service.
Our implementation of SSA supports range constraints on any numerical (float or integer) non-standard parameter, as well as strict match. The format follow the template adopted for some of the standard parameters (like e.g. REDSHIFT), itself inspired from the ISO8601 convention for date ranges. In general, the value has the form [low_bound]/[high_bound] (inclusive), and either of the bound may be omitted to specify an open range.