The first three (or five if an intersection) characters denote the fix at the end of the SID.
The number is incremented when the SID is modified (as Elevation says)
The final letter denotes the runway for the SID:
F EGLL 27L
G EGLL 27R
H EGLL 23
J EGLL 09R
K EGLL 09L
M EGKK 26L
P EGKK 08R
R EGSS 23
S EGSS 05
etc.