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C.)  PLF
Pounds per lineal foot is used to describe loads on walls or long members such as beams.  The beam receives an equal load for each foot of length.
Example: Beam 'A' has 2 sq ft of contributing load on each side (a tributary load). The load on each sq ft is 100 PSF.  Therefore 2 ft + 2 ft = a tributary width of 4 ft x 100 PSF = 400 PLF along the beam.
Note: Rafters and floor joists have a tributary load equal to their spacing, i.e., 12" on center, 16" on center, etc. Their PLF  =  PSF x  spacing in feet. To convert inches to feet, divide by 12.  Example: 16 inches / 12 = 1.333 ft.

D.)  UNIFORM LOAD
A uniform load is a continuous load along the entire length of a member and is expressed in PLF.  A partial uniform load is also expressed in PLF, but does not run the entire length of the member.
Note: The ends of joists and rafters bearing on a wall or beam each produce a small point load and when spaced 24"oc or less (in a uniform manner) they can be considered to produce uniform loading.

E.)  TRIBUTARY WIDTH
Tributary loading or tributary width is the accumulation of loads that are directed toward a particular structural member. 

Example: Tributary width is 7 ft + 5 ft = 12 ft.  If the load is 100 PSF, the load to the beam would be 12 ft x 100 PSF = 1200 PLF.  The left wall has 7 ft of tributary width and would receive a load of 700 PLF.  The right wall has 5 ft of tributary width and gets a load of 500 PLF.

Note:  No matter where the beam is located in relationship to the walls it will still have a tributary width of 12 ft which is one half the distance between the walls.  The tributary width to each outside wall will be one half the distance between the outside wall and beam.