BcalcOS uses SLiM (Simple Login Manager) as its display manager.
It provides the graphical login screen and starts the user’s graphical desktop session after authentication.
Why SLiM?
SLiM is simple, lightweight, and focused on doing its job without unnecessary complexity.
What Does SLiM Do?
When BcalcOS reaches graphical startup, SLiM presents the login screen after which it starts your configured graphical session.
In simplified form:
SysVinit → SLiM → Xfce
XLibre provides the X11 display server used by the graphical environment.
What Does This Mean for Users?
For most users, SLiM is simply the login screen. After authentication, it starts Xfce and gets you to your desktop.
Short Version
SLiM is the graphical gateway into BcalcOS.