http://www.militaryresearch.org/army_wwi_table_notes.htmhttp://www.militaryresearch.org/army_wwi_table_notes.htm "In each corps of six divisions, four divisions were combat division with their field artillery organized as shown in the below organization table. The other two divisions were depot (replacement) divisions organized as follows:"(go to link to read rest.)
also: http://www.history.army.mil/books/Lineage/M-F/chapter3.htmhttp://www.history.army.mil/books/Lineage/M-F/chapter3.htm "The organization of the sixteen National Army divisions also began in August when the designated division commanders, all Regular Army officers, and officer cadres reported to their respective training camps. Immediately thereafter the commanders established the 76th through the 91st Divisions and a depot brigade for each (Table 4).32 On 3 September the first draftees arrived. The depot brigades processed the new draftees while the divisions began a rigorous training program. Many of these men, however, quickly became fillers for National Guard and Regular Army units going overseas, one of the reasons that National Army divisions were unready for combat for many months.33"
The 81st Division was at Camp Jackson (as known then)so if they were in a "Depot"(replacement) brigade, they were in a new recruit or in training and then assigned to a regular unit (ie: Infantry, Train, Artillery etc.). So a Depot Brigade never left the Camp (or Fort), the men left the Depot Brigade to go overseas.