What is the starting point of Hellman's TMTO method? I understand what is happening and how the endpoint is computed but I'm still a bit unsure of where "SP" (Staring Point) came from. Are the starting points, randomly selected keys from the key-space n?

for example...
Given the diagram, yes they are randomly selected keys, $P$ is a fixed plaintext and the projection of the output to the keyspace gives you the next input as key to the cipher, thus creating a pseudorandom walk through the keyspace.