The apparent difference between DSP and ASP is that the former is quite an old guy whereas the latter is a boy Secondly, ASP has PSP on his shoulders where the ranker i.e DSP or SP (in some cases) has POLICE written on his shoulders beside ranks. Both have equal power but an ASP has definitely more respect and approach among senior PSP officers. DSP has respect too due to his long service in police.

Now coming to second part of your question. ASP, SP, SSP, DIG etc are ranks that are designated by the metal on your shoulders. Whereas, CPO, RPO, DPO and SDPO are posts. Every DPO is an SP or SSP but every SP or SSP is not DPO. Hope I cleared your query?