I've never been really happy with the English title we've given this set. Almost literally, 惜別球人 (Sekibetsu Kyuuhito) translates as "Regret of Departing Ball People" but that certainly makes no sense. Last year, I called the set the Farewell set. I've settled on "Commemorative Retirement Set" for this year's title, and am open to discussion on this.
Anyway, this is an annual tradition that BBM has issued including pretty much everyone who retired from professional baseball in the prior year. Issued as a box set, there are 36 base cards in the regular set, plus one additional card included in each box set.
I'm not sure why, but I haven't seen much opened product from this set available for sale. I was able to get my base card single from my usual card shop.