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In order to fully characterize the high-z systems probed by JWST comma it is essential that we prepare a robust rest-UV diagnostic toolkit to determine their physical and chemical properties. Crucially comma such diagnostics must incorporate the element that emits strongest in the UVdoublePoint carbon. Despite now having a suite of CIIICORCHETE_CLOSE comma HeII comma OIIICORCHETE_CLOSE comma and CIV UV emission lines from high-z analogue systems at our fingertips from CLASSY comma we are prevented from deriving accurate diagnostics because we lack a thorough understanding of the conditions affecting carbon emission in high-z environments. One key species is missingdoublePoint CORCHETE_OPENCIICORCHETE_CLOSE. Here we propose to observe the CORCHETE_OPENCIICORCHETE_CLOSE 2325A multiplet in a sample of 12 CLASSY galaxies using STIS-G230L spectroscopy comma thus building on pre-existing HST-COS spectroscopy to deliver essential insight into the diagnostic utility comma production comma and abundance of carbon. Our carefully chosen UV-emitting sample spans a wide range of high-z properties comma including low-metallicity comma high SFRs comma and high ionization parameters. Using this important multiplet we will (i) provide the first empirical calibration of a highly-promising UV-based metallicity diagnostic comma C23-HeII (ii) decipher the ionization structure of C in each system by harnessing all of carbon.s ionization stages (C+ comma C2+ comma C3+) (iii) derive highly accurate C-O abundances that do not rely on ionization correction factors (ICF) (iv) construct ICF grids for C to enable future C-O measurements in high-z systems. This study provides the first opportunity to thoroughly explore the origin of carbon in a range of high-z environments comma thus exploiting its full potential as a chemical and physical diagnostic in the JWST era.