In general, a rate transition exists between two blocks if their sample times differ, that is, if either of their sample-time vector components are different. The exceptions are:
A constant sample time ([Inf, 0]) never has a rate transition with any other rate.
A continuous sample time (black) and the fastest discrete rate (red) never has a rate transition if you use a fixed-step solver.
A variable sample time and fixed in minor step do not have a rate transition.
You can resolve rate transitions by inserting rate transition blocks and by using two diagnostic tools. For the single-tasking execution mode, the Single task rate transition diagnostic allows you to set the level of Simulink® rate transition messages. The Multitask rate transition diagnostic serves the same function for multitasking execution mode. These execution modes directly relate to the type of solver in use: Variable-step solvers are always single-tasking; fixed-step solvers may be explicitly set as single-tasking or multitasking.