There are many requirements that you have to meet in order to qualify for Naturalization. One such requirement is that you have to prove that you resided continuously in the US for five years before filing the citizenship form. If you are married to a US citizen and applying based on that marriage, then it is three years of continuous residence. Proving continuous residence simply means that you have maintained residence within the US for the required period of time as stated above.
If you are absent from the US for a lengthier period of time, it may disrupt the continuous residence requirement. If you are abroad for more than six months but lesser than a year, it may disrupt your continuous residence unless you can prove otherwise. If you are away from the US for a year or more, it means that you have disrupted your continuous residence even if you have a re-entry Permit. If you are abroad for a year or longer, you can come back as a permanent resident if you have a Re-entry Permit. But the time you were in the US before you left the country will not count toward your time in continuous residence.
If you come back within two years, some of the time you spent out of the country will count. In fact, the last 364 days of your time out of the country (one year minus one day) counts towards fulfilling the continuous residence requirement.
Another requirement is that you have to prove that you were physically present in the US for thirty months within the five year period before filing the citizenship form. If you are married to a US citizen and applying based on that marriage to a US citizen, then the requirement is eighteen months within the three year period. Adding to this requirement, you are required to prove that you resided for at least three months immediately prior to filing Form N-400 in the USCIS district or state where you claim to have residency.
Exceptions to the requirement
You can get an exception to the continuous residence requirement if you are working abroad for the US government (including the Military), contractors of the US government , a recognized American institution of research, a public international organization OR an organization designated under the International Immunities Act. To preserve your continuous residence for naturalization purposes while employed overseas by one of these recognized institutions, you are required to file Form N-470, Application to Preserve Residence for Naturalization Purposes with the designated USCIS office.
An organization has to get USCIS recognition as an American institution of research for the purpose of preserving the continuous residence status of its employees who intend to get naturalized and are employed overseas for an extended period of time. The simple difference between physical presence and continuous residence is that physical presence refers to the total number of days you were in the US during the period required for your naturalization.
Whereas continuous residence is the time you resided legally in the US without any single absence long enough to disrupt that continuity for naturalization purposes.