When you are using LEDES or ESIS Advocator as the external billing file format, the Ext. Code holds the UBTMS code.  They are listed here courtesy of the LEDES group, and may not be inclusive of any changes.

Task Codes
Litigation
L100	Case Assessment, Development and Administration.
L110	Fact Investigation/Development.
L120	Analysis/Strategy.
L130	Experts/Consultants.
L140	Document/File Management.
L150	Budgeting.
L160	Settlement/Non-Binding ADR.
L190	Other Case Assessment, Development and Administration.
L200	Pre-Trial Pleadings and Motions.
L210	Pleadings.
L220	Preliminary Injunctions/Provisional Remedies.
L230	Court Mandated Conferences.
L240	Dispositive Motions.
L250	Other Written Motions/Submissions.
L260	Class Action Certification and Notice.
L300	Discovery.
L310	Written Discovery.
L320	Document Production.
L330	Depositions.
L340	Expert Discovery.
L350	Discovery Motions.
L390	Other Discovery.
L400	Trial Preparation and Trial.
L410	Fact Witnesses.
L420	Expert Witnesses.
L430	Written Motions/Submissions.
L440	Other Trial Preparation and Support.
L450	Trial and Hearing Attendance.
L460	Post-Trial Motions and Submissions.
L470	Enforcement.
L500	Appeal.
L510	Appellate Motions and Submissions.
L520	Appellate Briefs.
L530	Oral Argument.
Counseling
C100	Fact Gathering.
C200	Researching Law.
C300	Analysis and Advice.
C400	Third Party Communication.
Project
P100	Project Administration.
P200	Fact Gathering/Due Diligence.
P210	Corporate Review.
P220	Tax.
P230	Environmental.
P240	Real and Personal Property.
P250	Employee/Labor.
P260	Intellectual Property.
P270	Regulatory Reviews.
P280	Other.
P300	Structure/Strategy/Analysis.
P400	Initial Document Preparation/Filing.
P500	Negotiation/Revision/Responses.
P600	Completion/Closing.
P700	Post-Completion/Post-Closing.
P800	Maintenance and Renewal.
Bankruptcy
B100	Administration.
B110	Case Administration.
B120	Asset Analysis and Recovery.
B130	Asset Disposition.
B140	Relief from Stay/Adequate Protection Proceedings.
B150	Meetings of and Communications with Creditors.
B160	Fee/Employment Applications.
B170	Fee/Employment Objections.
B180	Avoidance Action Analysis.
B185	Assumption/Rejection of Leases and Contracts.
B190	Other Contested Matters (excluding assumption/rejection motions).
B195	Non-Working Travel.
B200	Operations.
B210	Business Operations.
B220	Employee Benefits/Pensions.
B230	Financing/Cash Collections.
B240	Tax Issues.
B250	Real Estate.
B260	Board of Directors Matters.
B300	Claims and Plan.
B310	Claims Administration and Objections.
B320	Plan and Disclosure Statement (including Business Plan).
B400	Bankruptcy-Related Advice.
B410	General Bankruptcy Advice/Opinions.
B420	Restructurings.
Trademark
TR100	Assessment, Development, and Administration.
TR110	Fact Investigation and Development.
TR120	Analysis/Strategy.
TR130	Document/File Management.
TR140	Budgeting.
TR199	Other Assessment, Development.
TR200	Trademark Investigation and Analysis.
TR220	Registability Investigation.
TR230	Clearance Investigation.
TR240	Opposition Investigation.
TR250	Publication Watches.
TR260	Enforcement Investigation.
TR270	Status Investigation.
TR299	Other Trademark Investigation.
TR300	Domestic Trademark Application Preparation.
TR310	Application Preparation and Filing.
TR399	Other Domestic Trademark Aplication.
TR400	Domestic Trademark Prosecution and Renewal.
TR410	Affidavits, Petitions, Extensions.
TR420	Preliminary Amendment -Domestic.
TR430	Official Communication -Domestic.
TR440	Quasi-Judicial Administrative.
TR499	Other Trademark Prosecution.
TR500	International Trademark Application Preparation and Renewals.
TR510	Application Preparation and Filing.
TR599	Other International Trademark.
TR600	International Trademark Prosecution and Renewal.
TR610	Affidavits, Petitions, Extensions.
TR620	Preliminary Amendment.
TR630	Official Communication.
TR640	Quasi-Judicial Administrative.
TR699	Other Trademark Prosecution.
TR700	Other Trademark Related Tasks.
TR710	Opinion Preparation.
TR720	Portfolio Analysis and Management.
TR730	Assignments and Security Interests.
TR740	Licensing.
TR750	Domain Names -gTLDs.
TR760	Domain Names -ccTLDs.
TR770	Quasi-Judicial Administrative Proceedings.
TR799	Other Trademark Prosecution.
Activity Codes
A101	Plan and prepare for.
A102	Research.
A103	Draft/revise.
A104	Review/analyze.
A105	Communicate (in firm).
A106	Communicate (with client).
A107	Communicate (other outside counsel).
A108	Communicate (other external).
A109	Appear for/attend.
A110	Manage data/files.
A111	Other.
Expenses Codes
E101	Copying.
E102	Outside printing.
E103	Word processing.
E104	Facsimile.
E105	Telephone.
E106	Online research.
E107	Delivery services/messengers.
E108	Postage.
E109	Local travel.
E110	Out-of-town travel.
E111	Meals.
E112	Court fees.
E113	Subpoena fees.
E114	Witness fees.
E115	Deposition transcripts.
E116	Trial transcripts.
E117	Trial exhibits.
E118	Litigation support vendors.
E119	Experts.
E120	Private investigators.
E121	Arbitrators/mediators.
E122	Local counsel.
E123	Other professionals.
E124	Other.
E125	Translation Drawings Patent.
E126	Trademarks Records Searching.
E127	Monitoring Official Patent Fees.
E128	Renewal fees.
E129	Late fees.
E130	Post-Issuance Maintenance and Renewal Fees.
E131	Late Fees.

Combining codes

You can combine codes by joining them with a double colon, for example:

L110::A105

Will use code L110 as the task code and A105 as the activity code in the LEDES entry.