I'm not sure what a "complex real life simulator" is . A model is a description, often in mathematical terns, of what something is doing (that can also imbed how it does it). A simulation is an instantiation of a model, as in a program.
Simulations are always trying represent a "referrent," which is often what we perceive as reality, but can be something else - think of a video game that instantiates a model of a world with some make-believe physics.
Given that a simulation exists, one usually wants to understand the degree to which it reflects the thing that it is trying to model - it's fidelity, or resolution. Validation is the practice of documenting the degree to which a simulation produces results that agree with reality (or, the referrent).
Verification is the practice of documenting the extent to which good software practices were used, that the specification is met, that the algorithms and data have traceability.
Accreditation occurs when the authority reviews the V&V documentation and then uses her own judgement (and perhaps using subject matter experts) makes the decision that the simulation is fit for a particular use.
I'm not making this stuff up. You can find scads of information on the web about it. Not all of the definitions agree perfectly, but most are pretty similar to what I just gave you.
For reference: have you ever created a simulation?
