Under FAR 61.51, any time flown as the sole manipulator of the controls of an aircraft for which one is rated to act as PIC may be logged as PIC time. Many airlines and other employers have little interest in how much time one has spent manipulating the controls when they ask for an accounting of time spent "acting as the PIC". They want to know how many hours have been spent as the designated PIC. (part 1 definition) To "act as the PIC" is different than how the time is to be logged under 61.51.
Other operators, insurance companies, safety audit firms and the FAA itself make no distinction between the two definitions of "PIC time" for reporting purposes.
This presents somewhat of a quandary for certain FAA certificated pilots preparing themselves for submitting airline applications. To qualify for certain FAA issued certificates and ratings that specify minimum PIC times, any time spent in flight as "sole manipulator" may be logged as PIC as per 61.51. As stated previously, this same standard is commonly used by many other organizations with regard to reporting.
But then when the airline applicant goes to report their PIC time on an airline application, the airline specifies that for the purpose of the application, "PIC time means time during which the pilot was 'designated' or 'assigned' as 'THE PIC'." (there can only be one at a time!) So now the applicant has to go back and determine how much of their logged "PIC time" occurred when they were actually "THE PIC" The airline isn't interested in that 61.51 "sole manipulator" brand of "PIC time".
So here's what I suggest potential airline candidates do about logging PIC time. Do both! Log PIC according to 61.51 AND separately log time spent as "THE PIC". (part 1 definition) A separate column in the logbook or just asterisk the part 1 PIC time so it can be separated from the "sole manipulator" PIC time and added up at any time.
So to summarize, PIC time may be logged as long as it meets the stipulations of part 61.51. Some airlines and other operators may only be interested in how much of that time was spent "as "THE PIC". (part 1 definition)
I hope that helps.