On a GRS certificate or GemResearch Swisslab. It states that the ruby (being certified) is only subject to H treatment or H (a) is also acceptable.
According to GRS description
H - Enhanced by heat (no residues present) Commonly found in Mozambique Ruby
H(a) - Enhanced by heat – residues insignificant (indication of very small residues within fissures only). which is commonly found in Burma Ruby. This treatment is considered permanent enhancement. Extremely small residues of glass-like materials present in cavities or as isolated, extremely small residues in fissures that affect the weight of the gemstone less than approximately 0.001 carat.
H(b) - Enhanced by heat – minor residues are present (within fissures only). This category is similar to H(a) but has slightly more external material.
H / H(a) / H(b) - these are commonly accepted.
H(c) / H(d) - are considered lower quality gemstone
H(Be) - Beryllium treated gemstone