There's two types of injectables,
The first is,
Estradiol Valerate EV (which is not exactly like Beta estradiol, since the valeric acid
needs to be cleaved off before the body can use the estradiol, but this is
very easy for the body to do and thus there is little practical difference in effect
and risk)
I suppose they use it instead of injecting estradiol because the cracking of the
valeric acid is not all done at the same time which probably provides less of
a peak effect as putting estradiol into your bloodstream.
The second one is,
Estradiiol Cypionate (10 times more concentrated and longer to metabolise) EC
(EC because its so concentrated and longer lasting, has a higher propensity to be
abused by TS since its possible to get very very high serum levels of estrogen with it).
EC is also dirt cheap btw. At normal levels, it would actually be a better choice than
EV, but since its so often misused, doctor prever to give EV.