There are a number of important considerations when simulating from a more complex (hierarchical) model:
Re-simulating random effects / hierarchical structure: the first is that in a hierarchical model, several layers of stochasticity are aligned on top of each other. Specifically, in a GLMM, we have a lower level stochastic process (random effect), whose result enters into a higher level (e.g. Poisson distribution). For other hierarchical models such as state-space models, similar considerations apply. When simulating, we have to decide if we want to re-simulate all stochastic levels, or only a subset of those. For example, in a GLMM, it is common to only simulate the last stochastic level (e.g. Poisson) conditional on the fitted random effects.
For controlling how many levels should be re-simulated, the simulateResidual function allows to pass on parameters to the simulate function of the fitted model object. Please refer to the help of the different simulate functions (e.g. ?simulate.merMod) for details. For merMod (lme4) model objects, the relevant parameters are parameters are use.u and re.form
If the model is correctly specified, the simulated residuals should be flat regardles how many hierarchical levels we re-simulate. The most thorough procedure would therefore be to test all possible options. If testing only one option, I would recommend to re-simulate all levels, because this esentially tests the model structure as a whole. This is the default setting in the DHARMa package. A potential drawback is that re-simulating the lower-level random effects creates more variability, which may reduce power for detecing problems in the upper-level stochatic processes.
Integer responses: a second complication is the treatment of inter responses. Imaging we have observed a 0, and we predict 30% zeros - what is the quantile that we should display for the residual? To deal with this problem and maintain a uniform response, the option integerResponse adds a uniform noise from -0.5 to 0.5 on the simulated and observed response, which creates a uniform distribution - you can see this via hist(ecdf(runif(10000))(runif(10000))).
DHARMa will try to automatically if the fitted model has an integer or discrete distribution via the family argument. However, in some cases the family does not allow to uniquely identify the distribution type. For example, a tweedie distribution can be inter or continous. Therefore, DHARMa will additionally check the simulation results for repeated values, and will change the distribution type if repeated values are found (a message is displayed in this case).
Refitting or not: a third issue is how residuals are calculated. simulateResiduals has two options that are controlled by the refit parameter:
1. if refit = F (default), new data is simulated from the fitted model, and residuals are calculated by comparing the observed data to the new data
2. if refit = T, a parametric bootstrap is performed, meaning that the model is refit on the new data, and residuals are created by comparing observed residuals against refitted residuals
The second option is much slower, and only important for running tests that rely on comparing observed to simulated residuals, e.g. the testOverdispersion function
Residuals per group: In many situations, it can be useful to look at residuals per group, e.g. to see how much the model over / underpredicts per plot, year or subject. To do this, use recalculateResiduals, together with a grouping variable (see also help)