This article describes the different hydraulic boundary conditions available in Slide, and should make it more clear when each of the boundary conditions would be best used.
The figure below illustrates many of the different boundary conditions:
Constant head: ABC, EFG
No flow: HI
Free surface: CD
Note: The location of D is determined by the program as part of the solution, it is not defined by the user. It is the boundary between the saturated and unsaturated zones.
Seepage Face: DE
Descriptions of each boundary condition can be found below:

What is the Constant Head boundary condition?
Constant Head:
 Head at a given location does not change
 Implies an inexhaustible supply of water

What is the Nodal Flow boundary condition?
Nodal Flow:
 Constant rate of injection/extraction of water
 Head is calculated to produce specified flow (resulting head may be above ground surface or below base unless checked)
 This is analogous to a point load in FEM.

What is the Seepage Face boundary condition?
Seepage Face:
 Means that total head (h) must never exceed elevation head (z)
 Saturated zone intersects ground surface at atmospheric pressure and water discharges as evaporation or downhill film of flow
 General location of surface is known but its length is unknown prior to numerical analysis
 Modeled using the Unknown (Q=0 or P=0) boundary condition

What is the Infiltration boundary condition?
Infiltration:
 Volume of water entering or leaving an area (e.g. rainfall)
 Can only be applied to line segments and not individual nodes

What is the No Flow boundary condition?
No Flow:
 Similar to nodal flow except flow at boundary equals zero
 May be used to represent boundaries that yield negligible amounts of water to system
Also note the following tips regarding specifying boundary conditions:
 When a boundary has no boundary conditions assigned to it, that boundary is an impermeable boundary.
 When assigning a "Seepage Face condition" or "Unknown Boundary Condition" for a node or a segment of a boundary, that section cannot have pressure greater than zero. This means that whenever the water table rises to the level of that point, water can flow out of the model from that point (seep out).
 Discharge sections are best put inside boundaries and not on them.