What it is
Dashamsha divides each sign into ten equal parts of 3° each. After the navamsa it is probably the most consulted varga, since so many questions are about work and career.
How it is calculated
Rule: in an odd sign the first part belongs to the sign itself, then onward in order. In an even sign the first part belongs to the 9th sign from it, then onward.
Example: a planet is at Aries 17°. 17 ÷ 3 = 5.67, so the sixth part. Aries is odd, so the sixth sign from Aries: Virgo. In D10 the planet is in Virgo. At Taurus 17° the count would start from Capricorn, the 9th from Taurus, and the sixth part would be Gemini.
What tradition looks at in it
Parāśara links the dashamsha to achievements and position. Tradition looks at the D10 ascendant, the lord of the 10th house, and the Sun, Mercury and Saturn, which are associated with work and responsibility. It gives a glimpse of the kinds of work one is at ease in, for example fields that involve people, numbers or long-range planning.
How much the birth time matters
On average the ascendant moves about 1 degree every 4 minutes; some signs rise faster, some slower. One part of D10 is 3°, so a difference of roughly 12 minutes in birth time can change the D10 ascendant. The Moon crosses one part in about 5–6 hours. In your chart, ePandit shows next to every varga how many minutes of birth-time change would shift its ascendant. If your birth time may be off by 10–15 minutes, lean on the planets' D10 signs rather than the D10 ascendant.
How to read it calmly
Read D10 as a second look, not a verdict. Tradition first studies the 10th house and its lord in the main chart (D1), then the running dasha. D10 only helps check whether the same theme shows up here as well. A planet's sign in this varga decides nothing on its own. Career choices are shaped by your interests, skills and opportunities. D10 is a language that may help you think about them; it does not set the path.