(ז) לֹא מִקְרֵי הוֹרָאָה אֶלָּא כְּשֶׁמּוֹרֶה עַל מַעֲשֶׂה שֶׁבָּא לְפָנָיו, אֲבָל אִם שָׁאֲלוּ לְתַלְמִיד הֲלָכָה כְּדִבְרֵי מִי, יָכוֹל לוֹמַר מַה שֶּׁבְּדַעְתּוֹ, כֵּיוָן שֶׁאֵינוֹ מוֹרֶה עַל מַעֲשֶׂה שֶׁבָּא לְפָנָיו.
It is not called 'Horah' unless one is deciding the status of a specific incident that came before him. However, if one is asked a theoretical question of law, such as " who do we follow" this is permitted for him to decide....
(ח) לֹא מִקְרֵי הוֹרָאָה אֶלָּא בְּדָבָר שֶׁיֵּשׁ בּוֹ חִדּוּשׁ לַשּׁוֹאֵל, אֲבָל בְּהוֹרָאָה יְדוּעָה שֶׁהִיא פְּשׁוּטָה לַכֹּל, כְּגוֹן נוֹתֵן טַעַם לִפְגָם אוֹ לְבַטֵּל אִסוּר בְּשִׁשִּׁים וְכַיּוֹצֵא בָּאֵלּוּ, מֻתָּר.
"Horah" is only when one decides something that is new to the one asking. However, if one is just using well-known laws and applying them to an obvious situation, this is not Horah.
(ט) יֵשׁ מִי שֶׁכָּתַב, שֶׁכָּל הַכָּתוּב בַּסְפָרִים מִפִּסְקֵי הַגְּאוֹנִים יָכוֹל לְהוֹרוֹת בִּימֵי רַבּוֹ, רַק לֹא יוֹרֶה דָּבָר מִלִּבּוֹ וְלֹא יִסְמֹךְ עַל רְאָיוֹתָיו לְדַמּוֹת מִלְּתָא לְמִלְּתָא מֵעַצְמוֹ.
There are those that say that is one is using books of previous great sages as a source ( meaning the exact case was found there and he is merely quoting their opinion) one can do that during the life of his master. However, one should not make comparisions between two disparate cases.