The EWS1 form does seem to have lots of notes to refer to and some of them lead you to sources of reference.
For limited combustibility, for example under Option A you are referred to Note 6.
Note 6 states:
‘Note 6 – The term ‘limited combustibility’ is as defined in BS9991:2015’.
Now refer to BS9991:2015 Fire Safety in the Design, Management and Use of Residential Buildings – Code of practice.
If you don’t have a copy BS9991: 2015 defines material under limited combustibility under section 3.45 and states:
‘material of limited combustibility
a) a non-combustible material; or
b) any material of density 300 kg/m3 or more which, when tested in
accordance with BS 476-11, does not flame and the rise in temperature on
the furnace thermocouple is not more than 20 °C; or
c) any material with a non-combustible core of 8 mm thick or more, having
combustible facings (on one or both sides) not more than 0.5 mm thick; or
d) material classified as class A2-s3, d2 in accordance with BS EN 13501-1, when tested in accordance with BS EN ISO 1182 or BS EN ISO 1716 and BS EN 13823’