Demand Curve Equation Explanation

I've been reading (trying to brush up on my economics knowledge) and came across the equation: $q_D = a + b(P)$. The variable $a$ is explained as "factors other than price affecting demand."

I read up on non-price determinants of demand (e.g. income changes, preference changes, etc); but, what I've not been able to satisfactorily understand is how $a$ is calculated from those determinants.

So in the equation: $q_D = 6 - 0.5(P)$, what does $6$ represent and how was it determined?

In this demand curve, $a=6$ means that the autonomous demand for this good is of 6 units. That is, if the market price of that good was $0$, then people would demand 6 units.
It is reasonable to think that a positive income shock, or a shift in preferences making this good more desirable would make $a$ higher. I don't think there's any deeper meaning here, just that a downward sloping demand curve requires a positive intercept or else quantity demanded would be negative at any price $P$.