Maximum Power Transfer Theorem (MPTT):
This theorem is applicable only for linear networks i.e. the networks with R.L,C, transformer and linear controlled sources as elements.
The presence of dependent sources makes the network active and hence, MPTT is used for both active as well as passive networks.
This theorem is applicable only when load is variable, otherwise (i.e. load is fixed) choose the minimum internal impedance of the source, which results in maximum current through the fixed load and hence, a maximum power dissipation across the load.