so computershare in summary is just a place where you manage your shares, such as reinvestment plan, correct?
also when i buy other shares, they send me mail of the DRP, and quoting TFN.. if i done this on computershare, is it necessary to fill the mail, and mail it back to the Computershare investor services
Almost all companies use Link Marketing Services or Computershare for their registry services (I have come across another one for a small mining company I have shares in).
When you buy shares in a particular company for the first time, a few weeks later you will get a letter from the share registry company with some forms that you should fill in (TFN, DRP participation, Bank Details for dividend etc.). It's usually more convenient to fill the forms in there and then and mail them back in the postage free envelope provided. You can, however, go online at the share register website and enter your details there. If you want to just view your holding, your HIN and post code is usually all you need. If you want to change details, I think you have to register and get a user name and password. For subsequent purchases of this same share, the share registry uses the same details. So if you need to change anything later, it is easier to do it online.
Company specific options are, as you would expect, specific to that company's shareholding. If you buy shares in another company, you will get a set of forms for that company too, because it is feasible that you may want to have dividends paid into a different bank, or not participate in that company's DRP etc.
The only thing I have found with doing things online is that both of the big ones are very user unfriendly. If you want to change bank for all shareholdings, for instance, you must do it for each company individually, rather than selecting all companies together and making the change just once.