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)

Specified heads can also be specified using a combination of IBOUND and STRT in the Basic package.

 

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.