gear and brake cable sleeves can't be interchanged, but can it be explained why and what is the difference between what APPEARS to be two things that serve much the same function
AASHTA - recommend a read of this link for a complete answer.
Types of Cable Housing
Conventional Helical Housing
Cable housing (outer) is made of steel; a tightly-wrapped helix of steel wire: the plastic is a covering to protect it and the bike. It has no particular strength in tension but it cannot be compressed because the coils of wire are tight against one another.
Compressionless "Index-compatible" Housing
With the advent of indexed shifting
Shimano introduced "
S.I.S." housing, now widely copied by other manufacturers. This type of housing does not consist of a single helical-wound wire, but instead, it has a bundle of wires running pretty much straight along parallel to the housing, held in place by being sandwiched between the plastic housing liner and the plastic outer covering. "Compressionless" housing doesn't change length significantly as it is flexed, so the indexed shifter is able to communicate the correct setting to the derailleur.