From the link you provided:
D. Marital Union and Living in Marital Union
1. Married and Living in Marital Union
In general, all naturalization applicants filing on the basis of marriage to a U.S. citizen must continue to be the spouse of a U.S. citizen from the time of filing the naturalization application until the applicant takes the Oath of Allegiance. In addition, some spousal naturalization provisions require that the applicant “live in marital union” with his or her citizen spouse for at least 3 years immediately preceding the date of filing the naturalization application. [19] USCIS considers an applicant to “live in marital union” with his or her citizen spouse if the applicant and the citizen actually reside together.
An applicant does not meet the married and “living in marital union” requirements if:
The applicant is not residing with his or her U.S. citizen spouse at the time of filing or during the time in which the applicant is required to be living in marital union with the U.S. citizen spouse; or
The marital relationship is terminated at any time prior to taking the Oath of Allegiance.
If the applicant ceases to reside with his or her U.S. citizen spouse between the time of filing and the time at which the applicant takes the Oath of Allegiance, the officer should consider whether the applicant met the living in marital union requirement at the time of filing.
There are limited circumstances where an applicant may be able to establish that he or she is living in marital union with his or her citizen spouse even though the applicant does not actually reside with the citizen spouse. [20]
In all cases where it is applicable, the burden is on the applicant to establish that he or she has lived in marital union with his or her U.S. citizen spouse for the required period of time. [21]