how to setup Sequential Numbering in system profile at payables responsibility.

when we go line,my consultant create a accounting responsbility and system profile set Sequential Numbering equals 'always used'

and this responsbility only used to transfer journal to  GL,

in create payments or invoice workbach responsibility , the system profile set Sequential Numbering equals 'space'

i can't understand why i have to setup two responsbility and only different in Sequential Numbering values.