There are three types of boundary conditions in the Groundwater Flow process
Specified Head boundaries (which are examples of Dirichlet or first-type boundary conditions)
Specified Flux boundaries (which are examples of Neumann or second-type boundary conditions)
Head-Dependent Flux boundaries (which are examples of Robin or mixed boundary conditions)
Although MODFLOW does not require the use of any specific boundary condition package in order to run properly, it is not possible to simulate most real-world conditions properly without using at least one of them.