Existent

Halliday (1994: 142-3):

Figures Of Being: Semantic Characterisation

Halliday & Matthiessen (1999: 151-2):

In the limiting case, there is only one participant, the Existent; but generally there are two participants, the one being related by the process to the other. They may be being related by ascription, as Attribute to Carrier; or by identification, a rather complex relationship involving two pairs of participant roles: Identifier and Identified, and Token and Value. These latter intersect with each other, so there are two possible role combinations: 

(i) Identified/Token and Identifier/Value ['decoding']; 

(ii) Identifier/Token and Identified/Value ['encoding'].

Meteorological Processes: Existential, Material & Relational

Halliday & Matthiessen (2014: 309-10):

On this borderline between the ‘existential’ and the ‘material’ there is a special category of processes to do with the weather: meteorological processes like it’s raining, the wind’s blowing, there’s going to be a storm. Some are construed existentially, e.g. there was a storm/hurricane/breeze/gale/shower/blizzard. Some are construed as material events, e.g. the wind’s blowing, the sun’s shining, the clouds are coming down. Some are construed as relational attributives: it’s foggy/cloudy/misty/hot/humid/sunny/frosty; here the it can be interpreted as a Carrier, since it is possible to substitute the weather, the sky or the (time of) day. Finally, some are construed as it + a verb in the ‘present in present’ tense: it’s raining/hailing/snowing/freezing/pouring/drizzling/lightning/thundering.

Existent & Existential-Material Topology

Halliday & Matthiessen (2014: 309):

The entity or event which is said to exist is labelled, simply, Existent.  In principle, there can ‘exist’ any kind of phenomenon that can be construed as a ‘thing’: person, object, institution, abstraction; 

but also any action or event, as in is there going to be a storm?, there was another robbery in the street. And here the ‘existential’ merges into the ‘material’ type of clause: there is little difference in meaning between ‘existential’ there was a robbery and ‘material: creative’ a robbery took place (note the present tense a robbery is taking place).

Figures Of Being: Forms Of Participation Involved

Halliday & Matthiessen (1999: 151-2):

In the limiting case, there is only one participant, the Existent; but generally there are two participants, the one being related by the process to the other. They may be being related by ascription, as Attribute to Carrier; or by identification, a rather complex relationship involving two pairs of participant rôles: Identifier and Identified, and Token and Value. These latter intersect with each other, so that there are two possible rôle combinations: (i) Identified/Token and Identifier/Value [decoding]; (ii) Identifier/Token and Identified/Value [encoding].