Workflow related questions

Dear Experts,

Can you please clarify below workflow related questions?

1) No range for workflow is defined when customizing workflows have to be activated? T or F

2) I would like to know where we will maintain(t-code) no range for worfklow( the five digit number which will be generated by system internally?

3) Whether we can trigger the workflow using USERS_GEN or not?

Thanks

Ramakrishna