The Dasamsa is the tenth divisional chart: each sign is cut into ten parts of 3 degrees, and every planet is re-plotted into the sign its part falls in. In Brihat Parashara Hora Shastra ch. 6 it is the varga of career, profession and status, and it is read alongside the 10th house of the birth chart rather than in place of it.
Ten parts of 3 degrees each fill a sign. The counting start depends on parity rather than modality: in an odd sign the first part maps to the sign itself, and in an even sign it maps to the 9th sign from it. The remaining parts run forward from that start.
A worked example. A planet at 8 degrees of Aries, an odd sign, sits in the third part, so counting from Aries it lands in Gemini. The same 8 degrees in Taurus, an even sign, starts the count from Capricorn, the 9th from Taurus, and the third part from there is Pisces.
BPHS ch. 6 gives the D10 the subject of career and professional standing. A reading of it therefore looks at its own 10th house and 10th lord, and at which planets are strong in the division, exactly as the birth chart's 10th house is read - the division adds resolution rather than a new doctrine.
That resolution is the point of a varga. Two people with the same birth-chart 10th house can have quite different D10 charts, because a 3 degree part is crossed in about twelve minutes of clock time. The division separates careers that the sign-level chart cannot tell apart.
The tool re-plots the grahas and the ascendant into the D10, draws the chart from the dasamsa lagna, and reports each planet's sign, house and dignity in the division. Everything on it is arithmetic you can check against the construction rule above.
It does not tell you which profession to take. AstroZivi ships a reading only where a named text and verse stand behind it, and the classical assignment here is the subject of the chart, not a table of occupations. For career analysis that combines the D10 with the 10th house, the karakas and the running dasha, use the full kundli or the career report.
For educational purposes. Astrological interpretations are traditional and not a substitute for professional advice.