Why does it matter in which CPP file I add "template class X

It seems that there are (implicit) rules in which file I need to add a template class instantiation (template class X<Y>).

It seems that I need to do this in the X.cpp. If I do this in Y.cpp (where I include X.h) then I get linker errors (undefined reference, no compiler errors).

The namespaces are the same. So why does it matter where in place this template class ?