  The frame and scale dependence of the pair-term contribution to the
electromagnetic form factor of a spin-zero composite system of two-fermions is
studied within the Light Front. The form factor is evaluated from the
plus-component of the current in the Breit frame, using for the first time a
nonconstant, symmetric ansatz for the Bethe-Salpeter amplitude. The frame
dependence is analyzed by allowing a nonvanishing plus component of the
momentum transfer, while the dynamical scale is set by the masses of the
constituents and by mass and size of the composite system. A transverse
momentum distribution, associated with the Bethe-Salpeter amplitude, is
introduced which allows to define strongly and weakly relativistic systems. In
particular, for strongly relativistic systems, the pair term vanishes for the
Drell-Yan condition, while is dominant for momentum transfer along the
light-front direction. For a weakly relativistic system, fitted to the deuteron
scale, the pair term is negligible up to momentum transfers of 1(GeV/c)$^2$. A
comparison with results obtained within the Front-Form Hamiltonian dynamics
with a fixed number of constituents is also presented.
