The regulations discuss "the single security type method" to calculate cost basis (1.402(a)-1(b)(2)(ii)©)

Can someone explain when this method is used and provide an example of the calculation?

I specifically don't understand the following:
. When are securities sold to provide liquidity necessary for the trustee to exercise rights under the plan?
. What is the total amount credited to the account of a distributee (or portion that was available for investment in such securities)?
. What is the amount uninvested on the date of distribution?

Thanks