2:
In order to understand section 138 we must bear in mind what was established in 137:2, where it was clearly stated that

3:
Before we continue, in order to avoid confusion we must clarify our terms. It is common custom nowadays to refer to the passage that each honoree reads from the Torah as being a parsahah. (It is also customary to refer to the weekly lectionary from the Torah as parashah or parashat ha-shavu'a, but this is not relevant to our present discussion.) To refer thus the passage that each honoree reads is not only misleading but it is inaccurate. To avoid confusion we shall henceforth refer to such a passage as an Aliyyah. The term parashah on the other hand, refers to a section of text in the Sefer Torah. The text of the Torah is divided into sections which are indicated in one of two ways: either a unit of text is separated from the next unit by leaving the rest of the line blank before continuing with the next unit, or a space is left within the line which is approximately the width of nine letters. When the former sort of division is used the parashah is called 'open' and when the latter sort of division is used the parashah is termed 'closed'. Thus, the term Aliyyah is concerned with the division of the lectionary at the publc reading of the Torah, whereas the term parashah is concerned with the internal literary division of the text.

4:
The lectionary for Rosh Ĥodesh when it falls on a day other than Shabbat is Numbers 28:1-15, and the Gemara [Megillah 21b] tells us that on such a day the lectionary is divided up among four honorees. When we examine this text we find that it consists of three parashot - the first two are open parashot and the third is closed. The first parashah contains eight verses which describe the two daily sacrifices in the Bet Mikdash. The second parashah consists of two verses, which describe the additional sacrifice which was offered on Shabbat each week. The third parashah consists of five verses, and describes the additional sacrifices which were offered on Rosh Ĥodesh ever month. For your convenience, here is the text:

5:
We can now understand the following passage from the Gemara [Megillah 21b]:

6
The reason given for this rule is that we should not create a situation in which someone who was not present for the whole of the lectionary might get the mistaken impression that one or more of the honorees read less than three verses. When the lectionary was not fixed but each honoree read as much or as little as he wished it was important not to create a misunderstanding. Under normal circumstances if the reader has stopped when there are less than three verses until the end of a parshah or there are less than three verses from the beginning of a parashah he or she should be required to continue reading until they can stop without violating this rule or forcing the next honoree to violate this rule.

7:
Rabbi Moshe Isserles adds a note to the paragraph as penned by Rabbi Yosef Karo. (You will recall that Isserles is adding Ashkenazi custom to the Sefaradi bias of Karo. Only the last of this three comments requires comment from us. He quotes the custom, as originally reported by the students of Rabbi Me'ir of Rothenburg, that an effort should be made to avoid concluding an Aliyyah on a negative note. Obviously, it is not always possible to follow this recommendation.