Sprinkling the blood of the par (sacrificial bull) on the kaporet (ark cover) in the Holy of Holies as part of the Yom Kippur service is clearly commanded in the Torah (Vayikra 16:14). The Gemara on our daf quotes a baraita that teaches that in addition, there is a commandment is to sprinkle the blood in the direction of the parokhet (curtain), but not necessarily on the parokhet. This teaching brought Rabbi Elazar b’Rabbi Yosei to testify that on a visit to Rome he had the opportunity to examine the parokhet, and he saw drops of blood that he recognized as being from the Yom Kippur service. This was clear to him because the drops were in a straight row, and only the kohen gadol on Yom Kippur sprinkled the blood with such precision.
The Me’iri points out that only on Yom Kippur did the kohen gadol stand close to the parokhet when he did the zerikat ha-dam. Other sacrifices that had zerika on the parokhet were done with the officiating kohen standing behind the golden altar, a distance of more than twenty amot (cubits) from the parokhet, so it would have been impossible for the kohen to sprinkle the blood with any accuracy.
Rabbi Elazar was the son of the tanna Rabbi Yose ben Halafta, and lived in the last generation before the redaction of the Mishna by Rabbi Yehuda ha-Nasi. Rabbi Elazar was, apparently, the greatest of Rabbi Yose’s five sons, and already during his father’s lifetime he was recognized and honored by his generation.
During a difficult period for the Jews, Rabbi Elazar was part of a delegation to Rome together with Rabbi Shimon bar Yohai, which tried to get decrees against the Jews rescinded. Once in Rome they were miraculously given the opportunity to heal the Caesar’s daughter, who had fallen ill. They took advantage of this opportunity, and after successfully healing her, were given the opportunity to examine the Caesar’s coffers, which included the spoils of the Roman victory and sacking of the Land of Israel and the Temple. Rabbi Elazar’s examination of the Temple remains allowed him to return to the Sages with information about a number of the utensils from the mikdash, including the parokhet, the tzitz, etc.
For a description of the sprinkling of the blood in the Temple on Yom Kippur, see http://www.ou.org/torah/tt/5766/vayeira66/mikdash.htm