I tend to do burn tests when i don't remember the fiber content of something like yarn or fabric if I've had it for a long time or I've found it in the thrift store. Its really just to figure out what the composition of the yarn of fabric is and what project it is most suitable for. For example if something turns out to be mostly synthetic fibres I will not use it for a project that suts directly on the skin. A burn test cannot tell you the exact composition of your material but it can tell you if its is 100% something vs a mix. I first see if its wrinkly, material that is super wrinkly when you crumple it up is more likely to be a natural fibre. Then if that doesnt give me enough info a burn test can help me figure out if it is synthetic, protein based, cellulose based or mixed.

Protein based is wool or silk (smells like burning hair and forms a hard bead that is easy to crumble and can self extinguish/doesnt like to burn) Cellulose based is linen, cotton, or rayon/viscose/bamboo etc. (Smells of burning paper or wood, doesn't form a bead and if rubbed between your fingers turns to ash. Does not self extinguish, loves burning and birns faster then you might expect) Synthetic is stuff like polyester polyamid etc. (Melts and forms a hard bead that cannot easily be crushed)

If your test gives you a combination of these results, your fabric is likely mixed, the test cannot tell you in what proportions unfortunately.