frequency, rate, probability, proportion, ...

[choisy]

transitionProb: parameter is transitionProb, a fix rate transition moving per time step

OK, I think it's important we use correct terms here and don't make a huge messy confusion as epidemiologists traditionally do...

• A probability is between 0 and 1 and is the chance (or risk) that an event (death, infection, etc...) happens. In general this probability is defined over a period of time and will necessarily increase as the duration of this period of time increases.
• A rate is the frequency at which an event happens. It is thus always positive and always expressed per unit of time (like speed). Mathematically, it is the limit of the above-mentioned probability when the duration of the period of time tends towards zero (i.e. very small, i.e. instantaneous measure).
• A proportion is the ratio of a numerator and a denominator and by definition is between 0 and 1.

Here, it's not clear to me what transitionProb means. Do you mean that at every time step a fixed percentage of the left compartment transit to the right compartment? If yes, it needs to be explained explicitly so.