Ṣaḍ-bala is Pārāśara's quantitative model of graha-strength. Pre-classical and Tājika traditions tended to use single-criterion strength measures (just exaltation, or just dignity); BPHS introduced a six-component sum that's more interpretively robust. The six components are: sthāna-bala (positional strength based on rāśi placement and varga dignity); dig-bala (strength from the 'right' house for each graha); kāla-bala (temporal — day/night, paksha, etc.); naisargika-bala (inherent strength, fixed per graha); dṛk-bala (strength gained from being aspected); ceṣṭā-bala (strength from motion, primarily relevant to non-luminaries).
Each component is itself sub-divided. Sthāna-bala alone has five sub-components (uccha, mūla-trikoṇa, sapta-vargaja, kendrādi, drekkana). The computational details fill ten BPHS chapters (Ch. 17–27 inclusive, with Ch. 70 covering the synthesis). The /graha-bala engine surface computes the full ṣaḍ-bala for any chart.
Why ṣaḍ-bala matters interpretively: it converts the qualitative 'this graha is well-placed' / 'this graha is afflicted' into a numerical comparison. When Jupiter has 9.5 rūpas of total bala and Saturn has 5.2, the kāraka-of-jñāna (Jupiter) is clearly the dominant interpretive voice for that chart. The numbers are not predictions; they're prioritization signals.