1. Introduction

2. Photo ionization models
2.1. Star-forming galaxies
2.2. Active galactic Nuclei
2.3. Star-forming galaxies with an active galactic nucleus
2.4. Black bodies
2.5. PopIII star

3. UV line predictions

4. Applications of the UV diagrams
4.1. UV diagnostics to separate AGN from stellar photo ionization
4.2. UV diagrams to estimate ISM properties and star-formation age

5. C III Emitters found by VIMOS Ultra Deep Survey

6. The nature of the C III emitters from VUDS
6.1. Stack of all VUDS star-forming galaxies at z~3
6.2. Amorín et al. (2017)'s objects
6.3. Intermediate C III emitters with EW(C III) = 10-20 Å
6.4. Strongest C III emitters with EW(C III) > 20 Å

7. Discussion
7.1. The nature and physical properties of the C III emitters in VUDS sample of star-forming galaxies
7.2. On the origin of high C/O abundance ratios
7.3. Comparison with other high-redshift sources
7.4. Implications

8. Summary and conclusions