Feature summary (what you would like to be able to do and where):
Allow CheckUsers to in Special:InvestigateBlock specify a reason that is different for IPs and accounts.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
A CU blocking both IPs and accounts from an Investigation cannot make the connection obvious. This often includes making the reason used in the blocks on IPs different to those used on accounts. This can be because blocks on accounts link to a page that details the sockmaster or links to a account userpage. Having this information linked in the block summary for IPs is likely a violation of the disclosure agreements.

Benefits (why should this be implemented?):
Being able to specify a different reason would allow CUs to use Special:InvestigateBlock to block IPs and accounts. If this is not implemented either the reason used has to be vague or they have to use Special:InvestigateBlock for IPs and then accounts.