Basically what this line is going to disclose is depreciation allowable under "Alternative Minimum Tax (AMT)". AMT depreciation is calculated differently than regular federal income tax depreciation thus an adjustment must be made by a partner to properly calculate his/her share of AMT attributable from depreciable property in the partnership. Note that if the partnership does not have any depreciation deductions then this line will not apply.
See below for information on what method to use when computing depreciation under AMT.
For property placed in service before 1999, refigure depreciation for the AMT as follows (using the same convention used for the regular tax).
For section 1250 property (generally, residential rental and nonresidential real property), use the straight line method over 40 years.
For tangible property (other than section 1250 property) depreciated using the straight line method for the regular tax, use the straight line method over the property's class life. Use 12 years if the property has no class life.
For any other tangible property, use the 150% declining balance method, switching to the straight line method the first tax year it gives a larger deduction, over the property's AMT class life. Use 12 years if the property has no class life.
For property placed in service after 1998, refigure depreciation for the AMT only for property depreciated for the regular tax using the 200% declining balance method. For the AMT, use the 150% declining balance method, switching to the straight line method the first tax year it gives a larger deduction, and the same convention and recovery period used for the regular tax.
Figure the adjustment by subtracting the AMT deduction for depreciation from the regular tax deduction and enter the result on line 17a. If the AMT deduction is more than the regular tax deduction, enter the difference as a negative amount. Depreciation capitalized to inventory must also be refigured using the AMT rules. Include on this line the current year adjustment to income, if any, resulting from the difference.
I trust this provides the clarity you were looking for, please let me know if you have any further questions.